diff --git a/lib/common/appstart/localization_initialize.dart b/lib/common/appstart/localization_initialize.dart
index c773fd90..37d772b1 100644
--- a/lib/common/appstart/localization_initialize.dart
+++ b/lib/common/appstart/localization_initialize.dart
@@ -9,8 +9,8 @@ import 'package:kiwi/kiwi.dart';
 /// correct language
 ///
 class LocalizationInitialize {
-  PreferencesProvider _preferencesProvider;
-  String _languageCode;
+  PreferencesProvider? _preferencesProvider;
+  String? _languageCode;
 
   ///
   /// Initialize the localization using the provided language code
diff --git a/lib/common/data/database_access.dart b/lib/common/data/database_access.dart
index 8dfd9819..6b8a3851 100644
--- a/lib/common/data/database_access.dart
+++ b/lib/common/data/database_access.dart
@@ -4,15 +4,12 @@ import 'package:sqflite/sqflite.dart';
 
 class DatabaseAccess {
   static const String _databaseName = "Database.db";
-  static Database _databaseInstance;
+  static Database? _databaseInstance;
 
   static const String idColumnName = "id";
 
   Future<Database> get _database async {
-    if (_databaseInstance != null) return _databaseInstance;
-
-    _databaseInstance = await _initDatabase();
-    return _databaseInstance;
+    return _databaseInstance ??= await _initDatabase();
   }
 
   Future<Database> _initDatabase() async {
@@ -60,19 +57,20 @@ class DatabaseAccess {
   }
 
   Future<List<Map<String, dynamic>>> queryRows(String table,
-      {bool distinct,
-      List<String> columns,
-      String where,
-      List<dynamic> whereArgs,
-      String groupBy,
-      String having,
-      String orderBy,
-      int limit,
-      int offset}) async {
+      {bool? distinct,
+      List<String>? columns,
+      String? where,
+      List<dynamic>? whereArgs,
+      String? groupBy,
+      String? having,
+      String? orderBy,
+      int? limit,
+      int? offset}) async {
     Database db = await _database;
 
+    // TODO: [Leptopoda] is there a reason this is done? or at maybe use whereArgs.removeWhere()
     for (int i = 0; i < (whereArgs?.length ?? 0); i++) {
-      whereArgs[i] = whereArgs[i] ?? "";
+      whereArgs![i] = whereArgs[i] ?? "";
     }
 
     return await db.query(
@@ -89,33 +87,33 @@ class DatabaseAccess {
     );
   }
 
-  Future<int> queryRowCount(String table) async {
+  Future<int?> queryRowCount(String table) async {
     Database db = await _database;
     return Sqflite.firstIntValue(
         await db.rawQuery('SELECT COUNT(*) FROM $table'));
   }
 
-  Future<int> queryAggregator(String query, List<dynamic> arguments) async {
+  Future<int?> queryAggregator(String query, List<dynamic> arguments) async {
     Database db = await _database;
     return Sqflite.firstIntValue(await db.rawQuery(query, arguments));
   }
 
   Future<int> update(String table, Map<String, dynamic> row) async {
     Database db = await _database;
-    int id = row[idColumnName];
+    int? id = row[idColumnName];
     return await db
         .update(table, row, where: '$idColumnName = ?', whereArgs: [id]);
   }
 
-  Future<int> delete(String table, int id) async {
+  Future<int> delete(String table, int? id) async {
     Database db = await _database;
     return await db.delete(table, where: '$idColumnName = ?', whereArgs: [id]);
   }
 
   Future<int> deleteWhere(
     String table, {
-    String where,
-    List<dynamic> whereArgs,
+    String? where,
+    List<dynamic>? whereArgs,
   }) async {
     Database db = await _database;
     return await db.delete(
diff --git a/lib/common/data/database_entity.dart b/lib/common/data/database_entity.dart
index 121892b8..65967e9e 100644
--- a/lib/common/data/database_entity.dart
+++ b/lib/common/data/database_entity.dart
@@ -1,4 +1,4 @@
 abstract class DatabaseEntity {
   Map<String, dynamic> toMap();
-  void fromMap(Map<String, dynamic> map);
+  fromMap(Map<String, dynamic> map);
 }
diff --git a/lib/common/data/database_path_provider.dart b/lib/common/data/database_path_provider.dart
index 6d78badc..e5a5be00 100644
--- a/lib/common/data/database_path_provider.dart
+++ b/lib/common/data/database_path_provider.dart
@@ -2,7 +2,7 @@ import 'dart:io';
 
 import 'package:path_provider/path_provider.dart';
 import 'package:path/path.dart';
-import 'package:ios_app_group/ios_app_group.dart';
+import 'package:app_group_directory/app_group_directory.dart';
 
 Future<String> getDatabasePath(String databaseName) async {
   Directory documentsDirectory = await getApplicationDocumentsDirectory();
@@ -24,12 +24,13 @@ Future<String> _getiOSDatabasePathAndMigrate(
   // it must be saved in a group shared between the app module and the widget
   // module. "Migration" means that the database at the old path gets
   // copied to the new path
+  assert(Platform.isIOS);
 
-  var groupDirectory = await IosAppGroup.getAppGroupDirectory(
+  Directory? groupDirectory = await AppGroupDirectory.getAppGroupDirectory(
     'group.de.bennik2000.dhbwstudentapp',
   );
 
-  var newPath = join(groupDirectory.path, databaseName);
+  var newPath = join(groupDirectory!.path, databaseName);
 
   var migrateSuccess = await _migrateOldDatabase(oldPath, newPath);
 
diff --git a/lib/common/data/preferences/app_theme_enum.dart b/lib/common/data/preferences/app_theme_enum.dart
index 01aab80e..0c8be05f 100644
--- a/lib/common/data/preferences/app_theme_enum.dart
+++ b/lib/common/data/preferences/app_theme_enum.dart
@@ -1,9 +1,4 @@
-
 ///
 /// Enum which holds the possible themes the app can be displayed in
 ///
-enum AppTheme {
-  Dark,
-  Light,
-  System
-}
\ No newline at end of file
+enum AppTheme { Dark, Light, System }
diff --git a/lib/common/data/preferences/preferences_access.dart b/lib/common/data/preferences/preferences_access.dart
index 20cc4c9c..ea183eae 100644
--- a/lib/common/data/preferences/preferences_access.dart
+++ b/lib/common/data/preferences/preferences_access.dart
@@ -17,32 +17,26 @@ class PreferencesAccess {
       case int:
         await prefs.setInt(key, value as int);
         return;
+      default:
+        throw InvalidValueTypeException(T);
     }
-
-    throw InvalidValueTypeException(T);
   }
 
-  Future<T> get<T>(String key) async {
+  Future<T?> get<T>(String key) async {
     final SharedPreferences prefs = await SharedPreferences.getInstance();
 
-    T value;
-
     switch (T) {
       case bool:
-        value = prefs.getBool(key) as T;
-        break;
+        return prefs.getBool(key) as T?;
       case String:
-        value = prefs.getString(key) as T;
-        break;
+        return prefs.getString(key) as T?;
       case double:
-        value = prefs.getDouble(key) as T;
-        break;
+        return prefs.getDouble(key) as T?;
       case int:
-        value = prefs.getInt(key) as T;
-        break;
+        return prefs.getInt(key) as T?;
+      default:
+        return null;
     }
-
-    return value;
   }
 }
 
diff --git a/lib/common/data/preferences/preferences_provider.dart b/lib/common/data/preferences/preferences_provider.dart
index 5c6beaf5..61533ddd 100644
--- a/lib/common/data/preferences/preferences_provider.dart
+++ b/lib/common/data/preferences/preferences_provider.dart
@@ -51,23 +51,23 @@ class PreferencesProvider {
   }
 
   Future<void> setIsCalendarSyncEnabled(bool value) async {
-    await _preferencesAccess.set('isCalendarSyncEnabled', value);
+    await _preferencesAccess.set<bool>('isCalendarSyncEnabled', value);
   }
 
   Future<bool> isCalendarSyncEnabled() async {
-    return await _preferencesAccess.get('isCalendarSyncEnabled') ?? false;
+    return await _preferencesAccess.get<bool>('isCalendarSyncEnabled') ?? false;
   }
 
-  Future<void> setSelectedCalendar(Calendar selectedCalendar) async {
-    String selectedCalendarId = selectedCalendar?.id;
-    await _preferencesAccess.set(
-        'SelectedCalendarId', selectedCalendarId ?? '');
+  Future<void> setSelectedCalendar(Calendar? selectedCalendar) async {
+    final selectedCalendarId = selectedCalendar?.id ?? "";
+    await _preferencesAccess.set<String>(
+        'SelectedCalendarId', selectedCalendarId);
   }
 
-  Future<Calendar> getSelectedCalendar() async {
-    Calendar selectedCalendar;
-    String selectedCalendarId =
-        await _preferencesAccess.get('SelectedCalendarId') ?? null;
+  Future<Calendar?> getSelectedCalendar() async {
+    Calendar? selectedCalendar;
+    String? selectedCalendarId =
+        await _preferencesAccess.get<String>('SelectedCalendarId');
     if (selectedCalendarId == null) return null;
     List<Calendar> availableCalendars =
         await CalendarAccess().queryWriteableCalendars();
@@ -78,27 +78,27 @@ class PreferencesProvider {
   }
 
   Future<String> getRaplaUrl() async {
-    return await _preferencesAccess.get(RaplaUrlKey) ?? "";
+    return await _preferencesAccess.get<String>(RaplaUrlKey) ?? "";
   }
 
   Future<void> setRaplaUrl(String url) async {
-    await _preferencesAccess.set(RaplaUrlKey, url);
+    await _preferencesAccess.set<String>(RaplaUrlKey, url);
   }
 
   Future<bool> isFirstStart() async {
-    return await _preferencesAccess.get(IsFirstStartKey) ?? true;
+    return await _preferencesAccess.get<bool>(IsFirstStartKey) ?? true;
   }
 
   Future<void> setIsFirstStart(bool isFirstStart) async {
-    await _preferencesAccess.set(IsFirstStartKey, isFirstStart);
+    await _preferencesAccess.set<bool>(IsFirstStartKey, isFirstStart);
   }
 
-  Future<String> getLastUsedLanguageCode() async {
+  Future<String?> getLastUsedLanguageCode() async {
     return await _preferencesAccess.get<String>(LastUsedLanguageCode);
   }
 
   Future<void> setLastUsedLanguageCode(String languageCode) async {
-    await _preferencesAccess.set(LastUsedLanguageCode, languageCode);
+    await _preferencesAccess.set<String>(LastUsedLanguageCode, languageCode);
   }
 
   Future<bool> getNotifyAboutNextDay() async {
@@ -106,7 +106,7 @@ class PreferencesProvider {
   }
 
   Future<void> setNotifyAboutNextDay(bool value) async {
-    await _preferencesAccess.set(NotifyAboutNextDay, value);
+    await _preferencesAccess.set<bool>(NotifyAboutNextDay, value);
   }
 
   Future<bool> getNotifyAboutScheduleChanges() async {
@@ -115,7 +115,7 @@ class PreferencesProvider {
   }
 
   Future<void> setNotifyAboutScheduleChanges(bool value) async {
-    await _preferencesAccess.set(NotifyAboutScheduleChanges, value);
+    await _preferencesAccess.set<bool>(NotifyAboutScheduleChanges, value);
   }
 
   Future<bool> getDontShowRateNowDialog() async {
@@ -123,17 +123,22 @@ class PreferencesProvider {
   }
 
   Future<void> setDontShowRateNowDialog(bool value) async {
-    await _preferencesAccess.set(DontShowRateNowDialog, value);
+    await _preferencesAccess.set<bool>(DontShowRateNowDialog, value);
   }
 
   Future<void> storeDualisCredentials(Credentials credentials) async {
-    await _secureStorageAccess.set(DualisUsername, credentials.username ?? "");
-    await _secureStorageAccess.set(DualisPassword, credentials.password ?? "");
+    await _secureStorageAccess.set(DualisUsername, credentials.username);
+    await _secureStorageAccess.set(DualisPassword, credentials.password);
   }
 
-  Future<Credentials> loadDualisCredentials() async {
+  Future<Credentials?> loadDualisCredentials() async {
     var username = await _secureStorageAccess.get(DualisUsername);
     var password = await _secureStorageAccess.get(DualisPassword);
+
+    if (username == null ||
+        password == null ||
+        username.isEmpty ||
+        password.isEmpty) return null;
     return Credentials(username, password);
   }
 
@@ -147,30 +152,34 @@ class PreferencesProvider {
   }
 
   Future<void> setStoreDualisCredentials(bool value) async {
-    await _preferencesAccess.set(DualisStoreCredentials, value ?? false);
+    await _preferencesAccess.set<bool>(DualisStoreCredentials, value);
   }
 
-  Future<String> getLastViewedSemester() async {
+  Future<String?> getLastViewedSemester() async {
     return await _preferencesAccess.get<String>(LastViewedSemester);
   }
 
-  Future<void> setLastViewedSemester(String lastViewedSemester) async {
-    await _preferencesAccess.set(LastViewedSemester, lastViewedSemester);
+  Future<void> setLastViewedSemester(String? lastViewedSemester) async {
+    if (lastViewedSemester == null) return;
+    await _preferencesAccess.set<String>(
+        LastViewedSemester, lastViewedSemester);
   }
 
-  Future<String> getLastViewedDateEntryDatabase() async {
+  Future<String?> getLastViewedDateEntryDatabase() async {
     return await _preferencesAccess.get<String>(LastViewedDateEntryDatabase);
   }
 
-  Future<void> setLastViewedDateEntryDatabase(String value) async {
-    await _preferencesAccess.set<String>(LastViewedDateEntryDatabase, value);
+  Future<void> setLastViewedDateEntryDatabase(String? value) async {
+    await _preferencesAccess.set<String>(
+        LastViewedDateEntryDatabase, value ?? "");
   }
 
-  Future<String> getLastViewedDateEntryYear() async {
+  Future<String?> getLastViewedDateEntryYear() async {
     return await _preferencesAccess.get<String>(LastViewedDateEntryYear);
   }
 
-  Future<void> setLastViewedDateEntryYear(String value) async {
+  Future<void> setLastViewedDateEntryYear(String? value) async {
+    if (value == null) return;
     await _preferencesAccess.set<String>(LastViewedDateEntryYear, value);
   }
 
@@ -182,85 +191,88 @@ class PreferencesProvider {
     await _preferencesAccess.set<int>(ScheduleSourceType, value);
   }
 
-  Future<String> getIcalUrl() {
-    return _preferencesAccess.get(ScheduleIcalUrl);
+  Future<String?> getIcalUrl() {
+    return _preferencesAccess.get<String>(ScheduleIcalUrl);
   }
 
   Future<void> setIcalUrl(String url) {
-    return _preferencesAccess.set(ScheduleIcalUrl, url);
+    return _preferencesAccess.set<String>(ScheduleIcalUrl, url);
   }
 
-  Future<String> getMannheimScheduleId() {
-    return _preferencesAccess.get(MannheimScheduleId);
+  Future<String?> getMannheimScheduleId() {
+    return _preferencesAccess.get<String>(MannheimScheduleId);
   }
 
   Future<void> setMannheimScheduleId(String url) {
-    return _preferencesAccess.set(MannheimScheduleId, url);
+    return _preferencesAccess.set<String>(MannheimScheduleId, url);
   }
 
   Future<bool> getPrettifySchedule() async {
-    return await _preferencesAccess.get(PrettifySchedule) ?? true;
+    return await _preferencesAccess.get<bool>(PrettifySchedule) ?? true;
   }
 
   Future<void> setPrettifySchedule(bool value) {
-    return _preferencesAccess.set(PrettifySchedule, value);
+    return _preferencesAccess.set<bool>(PrettifySchedule, value);
   }
 
   Future<bool> getSynchronizeScheduleWithCalendar() async {
-    return await _preferencesAccess.get(SynchronizeScheduleWithCalendar) ??
+    return await _preferencesAccess
+            .get<bool>(SynchronizeScheduleWithCalendar) ??
         true;
   }
 
   Future<void> setSynchronizeScheduleWithCalendar(bool value) {
-    return _preferencesAccess.set(SynchronizeScheduleWithCalendar, value);
+    return _preferencesAccess.set<bool>(SynchronizeScheduleWithCalendar, value);
   }
 
   Future<bool> getDidShowWidgetHelpDialog() async {
-    return await _preferencesAccess.get(DidShowWidgetHelpDialog) ?? false;
+    return await _preferencesAccess.get<bool>(DidShowWidgetHelpDialog) ?? false;
   }
 
   Future<void> setDidShowWidgetHelpDialog(bool value) {
-    return _preferencesAccess.set(DidShowWidgetHelpDialog, value);
+    return _preferencesAccess.set<bool>(DidShowWidgetHelpDialog, value);
   }
 
   Future<void> set<T>(String key, T value) async {
+    if (value == null) return;
     return _preferencesAccess.set(key, value);
   }
 
-  Future<T> get<T>(String key) async {
-    return _preferencesAccess.get(key);
+  Future<T?> get<T>(String key) async {
+    return _preferencesAccess.get<T?>(key);
   }
 
   Future<int> getAppLaunchCounter() async {
-    return await _preferencesAccess.get("AppLaunchCount") ?? 0;
+    return await _preferencesAccess.get<int>("AppLaunchCount") ?? 0;
   }
 
   Future<void> setAppLaunchCounter(int value) async {
-    return await _preferencesAccess.set("AppLaunchCount", value);
+    return await _preferencesAccess.set<int>("AppLaunchCount", value);
   }
 
   Future<int> getNextRateInStoreLaunchCount() async {
-    return await _preferencesAccess.get("NextRateInStoreLaunchCount") ??
+    return await _preferencesAccess.get<int>("NextRateInStoreLaunchCount") ??
         RateInStoreLaunchAfter;
   }
 
   Future<void> setNextRateInStoreLaunchCount(int value) async {
-    return await _preferencesAccess.set("NextRateInStoreLaunchCount", value);
+    return await _preferencesAccess.set<int>(
+        "NextRateInStoreLaunchCount", value);
   }
 
   Future<bool> getDidShowDonateDialog() async {
-    return await _preferencesAccess.get("DidShowDonateDialog") ?? false;
+    return await _preferencesAccess.get<bool>("DidShowDonateDialog") ?? false;
   }
 
   Future<void> setDidShowDonateDialog(bool value) {
-    return _preferencesAccess.set("DidShowDonateDialog", value);
+    return _preferencesAccess.set<bool>("DidShowDonateDialog", value);
   }
 
   Future<bool> getHasPurchasedSomething() async {
-    return await _preferencesAccess.get("HasPurchasedSomething") ?? false;
+    return await _preferencesAccess.get<bool>("HasPurchasedSomething") ?? false;
   }
 
   Future<void> setHasPurchasedSomething(bool value) {
-    return _preferencesAccess.set("HasPurchasedSomething", value);
+    return _preferencesAccess.set<bool>("HasPurchasedSomething", value);
   }
 }
diff --git a/lib/common/data/preferences/secure_storage_access.dart b/lib/common/data/preferences/secure_storage_access.dart
index ec8e7442..76745504 100644
--- a/lib/common/data/preferences/secure_storage_access.dart
+++ b/lib/common/data/preferences/secure_storage_access.dart
@@ -7,7 +7,7 @@ class SecureStorageAccess {
     await _secureStorage.write(key: key, value: value);
   }
 
-  Future<String> get(String key) async {
+  Future<String?> get(String key) async {
     return await _secureStorage.read(key: key);
   }
 }
diff --git a/lib/common/i18n/localization_strings_de.dart b/lib/common/i18n/localization_strings_de.dart
index 252116e8..753959be 100644
--- a/lib/common/i18n/localization_strings_de.dart
+++ b/lib/common/i18n/localization_strings_de.dart
@@ -5,7 +5,8 @@ final de = {
   "settingsViewSourceCode": "Source code auf GitHub ansehen",
   "settingsCalendarSync": "Kalendersynchronisation",
   "calendarSyncPageTitle": "Kalender synchronisieren",
-  "calendarSyncPageSubtitle": "Wähle den Kalender aus, in den der Vorlesungsplan übertragen werden soll:",
+  "calendarSyncPageSubtitle":
+      "Wähle den Kalender aus, in den der Vorlesungsplan übertragen werden soll:",
   "calendarSyncPageEndSync": "Synchronisation beenden",
   "calendarSyncPageBeginSync": "Kalender synchronisieren",
   "applicationName": "DHBW Studenten App",
@@ -114,7 +115,8 @@ final de = {
   "dialogOk": "Ok",
   "dialogCancel": "Cancel",
   "dialogSetRaplaUrlTitle": "Rapla Url festlegen",
-  "dialogCalendarAccessNotGranted": "Wenn du den Vorlesungsplan mit deinem nativen Kalender synchronisieren möchtest, musst du in den Einstellungen den Kalenderzugriff erlauben.",
+  "dialogCalendarAccessNotGranted":
+      "Wenn du den Vorlesungsplan mit deinem nativen Kalender synchronisieren möchtest, musst du in den Einstellungen den Kalenderzugriff erlauben.",
   "dialogTitleCalendarAccessNotGranted": "Keine Zugriffsrechte",
   "scheduleEmptyStateSetUrl": "Konfigurieren",
   "scheduleEmptyStateBannerMessage":
diff --git a/lib/common/i18n/localizations.dart b/lib/common/i18n/localizations.dart
index c43f5842..fb82fdbc 100644
--- a/lib/common/i18n/localizations.dart
+++ b/lib/common/i18n/localizations.dart
@@ -6,10 +6,10 @@ import 'package:flutter/widgets.dart';
 
 class L {
   final Locale locale;
-  String _language;
+  late String _language;
 
   L(this.locale) {
-    _language = locale?.languageCode?.substring(0, 2);
+    _language = locale.languageCode.substring(0, 2);
 
     if (!_localizedValues.containsKey(_language)) {
       _language = "en";
@@ -84,7 +84,8 @@ class L {
 
   String get calendarSyncPageEndSync => _getValue("calendarSyncPageEndSync");
 
-  String get calendarSyncPageBeginSync => _getValue("calendarSyncPageBeginSync");
+  String get calendarSyncPageBeginSync =>
+      _getValue("calendarSyncPageBeginSync");
 
   String get notificationScheduleChangedNewClass =>
       _getValue("notificationScheduleChangedNewClass");
@@ -283,9 +284,11 @@ class L {
 
   String get dialogSetRaplaUrlTitle => _getValue("dialogSetRaplaUrlTitle");
 
-  String get dialogCalendarAccessNotGranted => _getValue("dialogCalendarAccessNotGranted");
+  String get dialogCalendarAccessNotGranted =>
+      _getValue("dialogCalendarAccessNotGranted");
 
-  String get dialogTitleCalendarAccessNotGranted => _getValue("dialogTitleCalendarAccessNotGranted");
+  String get dialogTitleCalendarAccessNotGranted =>
+      _getValue("dialogTitleCalendarAccessNotGranted");
 
   String get scheduleEmptyStateSetUrl => _getValue("scheduleEmptyStateSetUrl");
 
@@ -403,7 +406,7 @@ class L {
   String get filterTitle => _getValue("filterTitle");
 
   static L of(BuildContext context) {
-    return Localizations.of<L>(context, L);
+    return Localizations.of<L>(context, L)!;
   }
 
   static final Map<String, Map<String, String>> _localizedValues = {
@@ -412,7 +415,7 @@ class L {
   };
 
   String _getValue(String key) {
-    return _localizedValues[_language][key] ?? "";
+    return _localizedValues[_language]![key] ?? "";
   }
 
   String getValue(String key) => _getValue(key);
diff --git a/lib/common/iap/in_app_purchase_helper.dart b/lib/common/iap/in_app_purchase_helper.dart
index efe60867..3d9d50cd 100644
--- a/lib/common/iap/in_app_purchase_helper.dart
+++ b/lib/common/iap/in_app_purchase_helper.dart
@@ -20,7 +20,7 @@ enum PurchaseResultEnum {
 }
 
 typedef PurchaseCompletedCallback = Function(
-  String productId,
+  String? productId,
   PurchaseResultEnum result,
 );
 
@@ -33,10 +33,10 @@ class InAppPurchaseHelper {
 
   final PreferencesProvider _preferencesProvider;
 
-  StreamSubscription _purchaseUpdatedSubscription;
-  StreamSubscription _purchaseErrorSubscription;
+  StreamSubscription? _purchaseUpdatedSubscription;
+  StreamSubscription? _purchaseErrorSubscription;
 
-  PurchaseCompletedCallback _purchaseCallback;
+  PurchaseCompletedCallback? _purchaseCallback;
 
   InAppPurchaseHelper(this._preferencesProvider);
 
@@ -77,7 +77,7 @@ class InAppPurchaseHelper {
       return PurchaseResultEnum.Success;
     } on PlatformException catch (_) {
       if (_purchaseCallback != null) {
-        _purchaseCallback(id, PurchaseResultEnum.Error);
+        _purchaseCallback!(id, PurchaseResultEnum.Error);
       }
 
       return PurchaseResultEnum.Error;
@@ -95,25 +95,23 @@ class InAppPurchaseHelper {
       return PurchaseStateEnum.NotPurchased;
     }
 
-    var allPurchases = [];
-
     try {
-      allPurchases =
+      var allPurchases =
           await FlutterInappPurchase.instance.getAvailablePurchases();
-    } on Exception catch (_) {
-      return PurchaseStateEnum.Unknown;
-    }
 
-    var productIdPurchases =
-        allPurchases.where((element) => element.productId == id);
+      var productIdPurchases =
+          allPurchases?.where((element) => element.productId == id);
+
+      if (productIdPurchases?.isNotEmpty ?? false) {
+        return productIdPurchases!.any((element) => _isPurchased(element))
+            ? PurchaseStateEnum.Purchased
+            : PurchaseStateEnum.NotPurchased;
+      }
 
-    if (productIdPurchases.isEmpty) {
       return PurchaseStateEnum.NotPurchased;
+    } on Exception catch (_) {
+      return PurchaseStateEnum.Unknown;
     }
-
-    return productIdPurchases.any((element) => _isPurchased(element))
-        ? PurchaseStateEnum.Purchased
-        : PurchaseStateEnum.NotPurchased;
   }
 
   ///
@@ -124,7 +122,9 @@ class InAppPurchaseHelper {
     _purchaseCallback = callback;
   }
 
-  Future<void> _completePurchase(PurchasedItem item) async {
+  Future<void> _completePurchase(PurchasedItem? item) async {
+    if (item == null) return;
+
     var purchaseResult = _purchaseResultFromItem(item);
 
     _purchaseCallback?.call(item.productId, purchaseResult);
@@ -152,48 +152,40 @@ class InAppPurchaseHelper {
   }
 
   PurchaseResultEnum _purchaseResultFromItem(PurchasedItem item) {
-    var purchaseResult = PurchaseResultEnum.Error;
-
     if (Platform.isAndroid) {
       switch (item.purchaseStateAndroid) {
         case PurchaseState.pending:
-          purchaseResult = PurchaseResultEnum.Pending;
-          break;
+          return PurchaseResultEnum.Pending;
+
         case PurchaseState.purchased:
-          purchaseResult = PurchaseResultEnum.Success;
-          break;
+          return PurchaseResultEnum.Success;
         case PurchaseState.unspecified:
-          purchaseResult = PurchaseResultEnum.Error;
-          break;
+        default:
+          return PurchaseResultEnum.Error;
       }
     } else if (Platform.isIOS) {
       switch (item.transactionStateIOS) {
         case TransactionState.purchasing:
-          purchaseResult = PurchaseResultEnum.Pending;
-          break;
+          return PurchaseResultEnum.Pending;
         case TransactionState.purchased:
-          purchaseResult = PurchaseResultEnum.Success;
-          break;
-        case TransactionState.failed:
-          purchaseResult = PurchaseResultEnum.Error;
-          break;
+          return PurchaseResultEnum.Success;
         case TransactionState.restored:
-          purchaseResult = PurchaseResultEnum.Success;
-          break;
+          return PurchaseResultEnum.Success;
         case TransactionState.deferred:
-          purchaseResult = PurchaseResultEnum.Pending;
-          break;
+          return PurchaseResultEnum.Pending;
+        case TransactionState.failed:
+        default:
+          return PurchaseResultEnum.Error;
       }
     }
-
-    return purchaseResult;
+    return PurchaseResultEnum.Error;
   }
 
   Future<bool> _hasFinishedTransaction(PurchasedItem item) async {
     if (item.isAcknowledgedAndroid ?? false) return true;
 
     return await _preferencesProvider
-            .get("purchase_${item.productId}_finished") ??
+            .get<bool>("purchase_${item.productId}_finished") ??
         false;
   }
 
@@ -206,7 +198,7 @@ class InAppPurchaseHelper {
       return;
     }
 
-    List<PurchasedItem> purchasedItems = [];
+    List<PurchasedItem>? purchasedItems;
 
     if (Platform.isAndroid) {
       purchasedItems =
@@ -216,20 +208,23 @@ class InAppPurchaseHelper {
           await FlutterInappPurchase.instance.getPendingTransactionsIOS();
     }
 
-    print("Found ${purchasedItems.length} pending purchases");
+    if (purchasedItems != null) {
+      print("Found ${purchasedItems.length} pending purchases");
 
-    purchasedItems.forEach(_completePurchase);
+      purchasedItems.forEach(_completePurchase);
+    }
   }
 
-  void _onPurchaseError(PurchaseResult event) {
+  void _onPurchaseError(PurchaseResult? event) {
     print("Failed to purchase:");
-    print(event.message);
-    print(event.debugMessage);
+    print(event?.message);
+    print(event?.debugMessage);
 
     _purchaseCallback?.call(null, PurchaseResultEnum.Error);
   }
 
-  bool _isConsumable(String id) {
+  // TODO: [Leptopdoa] remove this¿?
+  bool _isConsumable(String? id) {
     return false;
   }
 
diff --git a/lib/common/iap/in_app_purchase_manager.dart b/lib/common/iap/in_app_purchase_manager.dart
index c56b8f96..75b7d540 100644
--- a/lib/common/iap/in_app_purchase_manager.dart
+++ b/lib/common/iap/in_app_purchase_manager.dart
@@ -22,7 +22,7 @@ class InAppPurchaseManager {
   void _initialize() async {
     addPurchaseCallback(
       InAppPurchaseHelper.WidgetProductId,
-      (String productId, PurchaseResultEnum result) =>
+      (String? productId, PurchaseResultEnum result) =>
           _setWidgetEnabled(result == PurchaseResultEnum.Success),
     );
 
@@ -32,8 +32,8 @@ class InAppPurchaseManager {
     try {
       await _inAppPurchaseHelper.initialize();
       await _restorePurchases();
-    }
-    catch (ex) {
+    } catch (ex) {
+      // TODO: [Leptopoda] disable purchases or show error message in settings when initialization was not sucessfull (i.e. no play services)
       print("Failed to initialize in app purchase!");
     }
   }
@@ -61,19 +61,21 @@ class InAppPurchaseManager {
     await buyWidget();
   }
 
-  void _purchaseCompletedCallback(String productId, PurchaseResultEnum result) {
+  // TODO: [Leptopoda] better nullseafety
+  void _purchaseCompletedCallback(
+      String? productId, PurchaseResultEnum result) {
     if (purchaseCallbacks.containsKey(productId)) {
-      var callback = purchaseCallbacks[productId] ?? [];
+      var callback = purchaseCallbacks[productId!];
 
-      callback.forEach((element) {
+      callback?.forEach((element) {
         element(productId, result);
       });
     }
 
     if (purchaseCallbacks.containsKey("*")) {
-      var callback = purchaseCallbacks["*"] ?? [];
+      var callback = purchaseCallbacks["*"];
 
-      callback.forEach((element) {
+      callback?.forEach((element) {
         element(productId, result);
       });
     }
@@ -91,25 +93,21 @@ class InAppPurchaseManager {
   /// for all product ids, pass null or "*" as productId
   ///
   void addPurchaseCallback(
-    String productId,
+    String? productId,
     PurchaseCompletedCallback callback,
   ) {
     if (productId == null) {
       productId = "*";
     }
 
-    if (!purchaseCallbacks.containsKey(productId)) {
-      purchaseCallbacks[productId] = [];
-    }
-
-    purchaseCallbacks[productId].add(callback);
+    purchaseCallbacks[productId]?.add(callback);
   }
 
   ///
   /// Removes a callback which was registered using [addPurchaseCallback]
   ///
   void removePurchaseCallback(
-    String productId,
+    String? productId,
     PurchaseCompletedCallback callback,
   ) {
     if (productId == null) {
diff --git a/lib/common/logging/crash_reporting.dart b/lib/common/logging/crash_reporting.dart
index ad0dde6b..0e863cfa 100644
--- a/lib/common/logging/crash_reporting.dart
+++ b/lib/common/logging/crash_reporting.dart
@@ -1,7 +1,7 @@
 import 'package:firebase_crashlytics/firebase_crashlytics.dart';
 import 'package:flutter/foundation.dart';
 
-Future<void> reportException(ex, StackTrace trace) async {
+Future<void> reportException(ex, StackTrace? trace) async {
   if (kReleaseMode) {
     print("Reporting exception to crashlytics: $ex with stack trace $trace");
     await FirebaseCrashlytics.instance.recordError(ex, trace);
diff --git a/lib/common/ui/colors.dart b/lib/common/ui/colors.dart
index 181b7c65..1cea4e06 100644
--- a/lib/common/ui/colors.dart
+++ b/lib/common/ui/colors.dart
@@ -68,7 +68,7 @@ Color colorNoConnectionForeground() => Colors.white;
 class ColorPalettes {
   ColorPalettes._();
 
-  static ThemeData buildTheme(AppTheme theme) {
+  static ThemeData buildTheme(AppTheme? theme) {
     if (theme == AppTheme.System) {
       theme = PlatformUtil.platformBrightness() == Brightness.light
           ? AppTheme.Light
@@ -93,14 +93,14 @@ class ColorPalettes {
     return themeData.copyWith(
       snackBarTheme: themeData.snackBarTheme.copyWith(
         backgroundColor: isDark ? Color(0xff363635) : Color(0xfffafafa),
-        contentTextStyle: themeData.textTheme.bodyText1.copyWith(
+        contentTextStyle: themeData.textTheme.bodyText1!.copyWith(
           color:
-              isDark ? Color(0xffe4e4e4) : themeData.textTheme.bodyText1.color,
+              isDark ? Color(0xffe4e4e4) : themeData.textTheme.bodyText1!.color,
         ),
       ),
       textButtonTheme: TextButtonThemeData(
         style: TextButton.styleFrom(
-          primary: ColorPalettes.main,
+          foregroundColor: ColorPalettes.main,
           padding: EdgeInsets.symmetric(horizontal: 16.0),
           shape: const RoundedRectangleBorder(
             borderRadius: BorderRadius.all(Radius.circular(4.0)),
diff --git a/lib/common/ui/custom_icons_icons.dart b/lib/common/ui/custom_icons_icons.dart
index 759832fb..39da89d1 100644
--- a/lib/common/ui/custom_icons_icons.dart
+++ b/lib/common/ui/custom_icons_icons.dart
@@ -11,7 +11,7 @@
 ///      fonts:
 ///       - asset: fonts/CustomIcons.ttf
 ///
-/// 
+///
 /// * Font Awesome 4, Copyright (C) 2016 by Dave Gandy
 ///         Author:    Dave Gandy
 ///         License:   SIL ()
@@ -23,7 +23,8 @@ class CustomIcons {
   CustomIcons._();
 
   static const _kFontFam = 'CustomIcons';
-  static const _kFontPkg = null;
+  static const dynamic _kFontPkg = null;
 
-  static const IconData logout = IconData(0xe800, fontFamily: _kFontFam, fontPackage: _kFontPkg);
+  static const IconData logout =
+      IconData(0xe800, fontFamily: _kFontFam, fontPackage: _kFontPkg);
 }
diff --git a/lib/common/ui/notification_api.dart b/lib/common/ui/notification_api.dart
index 9810bf1e..ae714d84 100644
--- a/lib/common/ui/notification_api.dart
+++ b/lib/common/ui/notification_api.dart
@@ -20,6 +20,7 @@ class NotificationApi {
     );
 
     var initializationSettingsIOS = IOSInitializationSettings(
+      // TODO: [Leptopoda] the below always returns null so why register it?
       onDidReceiveLocalNotification: onDidReceiveLocalNotification,
     );
 
@@ -37,7 +38,7 @@ class NotificationApi {
   ///
   /// Show a notification with the given title and message
   ///
-  Future showNotification(String title, String message, [int id]) async {
+  Future showNotification(String title, String? message, [int? id]) async {
     var androidPlatformChannelSpecifics = AndroidNotificationDetails(
       'Notifications',
       'Notifications',
@@ -72,10 +73,10 @@ class NotificationApi {
   }
 
   Future onDidReceiveLocalNotification(
-          int id, String title, String body, String payload) =>
+          int id, String? title, String? body, String? payload) =>
       Future.value();
 
-  Future selectNotification(String payload) => Future.value();
+  Future selectNotification(String? payload) => Future.value();
 }
 
 ///
@@ -83,7 +84,8 @@ class NotificationApi {
 ///
 class VoidNotificationApi implements NotificationApi {
   @override
-  FlutterLocalNotificationsPlugin get _localNotificationsPlugin => null;
+  FlutterLocalNotificationsPlugin get _localNotificationsPlugin =>
+      throw UnimplementedError();
 
   @override
   Future<void> initialize() {
@@ -92,17 +94,17 @@ class VoidNotificationApi implements NotificationApi {
 
   @override
   Future onDidReceiveLocalNotification(
-      int id, String title, String body, String payload) {
+      int id, String? title, String? body, String? payload) {
     return Future.value();
   }
 
   @override
-  Future selectNotification(String payload) {
+  Future selectNotification(String? payload) {
     return Future.value();
   }
 
   @override
-  Future showNotification(String title, String message, [int id]) {
+  Future showNotification(String title, String? message, [int? id]) {
     return Future.value();
   }
 }
diff --git a/lib/common/ui/schedule_entry_type_mappings.dart b/lib/common/ui/schedule_entry_type_mappings.dart
index 9c162016..f934ce95 100644
--- a/lib/common/ui/schedule_entry_type_mappings.dart
+++ b/lib/common/ui/schedule_entry_type_mappings.dart
@@ -25,14 +25,14 @@ final Map<ScheduleEntryType, TextDelegate> scheduleEntryTypeTextMapping = {
 
 Color scheduleEntryTypeToColor(
   BuildContext context,
-  ScheduleEntryType type,
+  ScheduleEntryType? type,
 ) {
-  return scheduleEntryTypeColorMapping[type](context);
+  return scheduleEntryTypeColorMapping[type!]!(context);
 }
 
 String scheduleEntryTypeToReadableString(
   BuildContext context,
-  ScheduleEntryType type,
+  ScheduleEntryType? type,
 ) {
-  return scheduleEntryTypeTextMapping[type](context);
+  return scheduleEntryTypeTextMapping[type!]!(context);
 }
diff --git a/lib/common/ui/text_styles.dart b/lib/common/ui/text_styles.dart
index f4ec8f22..94b731db 100644
--- a/lib/common/ui/text_styles.dart
+++ b/lib/common/ui/text_styles.dart
@@ -1,56 +1,56 @@
 import 'package:flutter/material.dart';
 
-TextStyle textStyleDailyScheduleEntryWidgetProfessor(BuildContext context) =>
+TextStyle? textStyleDailyScheduleEntryWidgetProfessor(BuildContext context) =>
     Theme.of(context).textTheme.subtitle2;
 
 TextStyle textStyleDailyScheduleEntryWidgetTitle(BuildContext context) =>
     Theme.of(context)
         .textTheme
-        .headline4
-        .copyWith(color: Theme.of(context).textTheme.headline6.color);
+        .headline4!
+        .copyWith(color: Theme.of(context).textTheme.headline6!.color);
 
 TextStyle textStyleDailyScheduleEntryWidgetType(BuildContext context) =>
-    Theme.of(context).textTheme.bodyText2.copyWith(
+    Theme.of(context).textTheme.bodyText2!.copyWith(
           fontWeight: FontWeight.w300,
           letterSpacing: 0.15,
         );
 
 TextStyle textStyleDailyScheduleEntryWidgetTimeStart(BuildContext context) =>
-    Theme.of(context).textTheme.headline5.copyWith(
+    Theme.of(context).textTheme.headline5!.copyWith(
           fontWeight: FontWeight.w600,
           letterSpacing: 0.15,
         );
 
-TextStyle textStyleDailyScheduleEntryWidgetTimeEnd(BuildContext context) =>
+TextStyle? textStyleDailyScheduleEntryWidgetTimeEnd(BuildContext context) =>
     Theme.of(context).textTheme.subtitle2;
 
 TextStyle textStyleDailyScheduleCurrentDate(BuildContext context) =>
-    Theme.of(context).textTheme.headline4.copyWith(
-          color: Theme.of(context).textTheme.headline5.color,
+    Theme.of(context).textTheme.headline4!.copyWith(
+          color: Theme.of(context).textTheme.headline5!.color,
         );
-TextStyle textStyleDailyScheduleNoEntries(BuildContext context) =>
+TextStyle? textStyleDailyScheduleNoEntries(BuildContext context) =>
     Theme.of(context).textTheme.headline5;
 
 TextStyle textStyleScheduleEntryWidgetTitle(BuildContext context) =>
-    Theme.of(context).textTheme.bodyText1.copyWith(
+    Theme.of(context).textTheme.bodyText1!.copyWith(
           fontWeight: FontWeight.normal,
         );
 
-TextStyle textStyleScheduleEntryBottomPageTitle(BuildContext context) =>
+TextStyle? textStyleScheduleEntryBottomPageTitle(BuildContext context) =>
     Theme.of(context).textTheme.subtitle2;
 
-TextStyle textStyleScheduleEntryBottomPageTimeFromTo(BuildContext context) =>
+TextStyle? textStyleScheduleEntryBottomPageTimeFromTo(BuildContext context) =>
     Theme.of(context).textTheme.caption;
 
-TextStyle textStyleScheduleEntryBottomPageTime(BuildContext context) =>
+TextStyle? textStyleScheduleEntryBottomPageTime(BuildContext context) =>
     Theme.of(context).textTheme.headline5;
 
 TextStyle textStyleScheduleEntryBottomPageType(BuildContext context) =>
-    Theme.of(context).textTheme.bodyText1.copyWith(
+    Theme.of(context).textTheme.bodyText1!.copyWith(
           fontWeight: FontWeight.w300,
         );
 
 TextStyle textStyleScheduleWidgetColumnTitleDay(BuildContext context) =>
-    Theme.of(context).textTheme.subtitle2.copyWith(
+    Theme.of(context).textTheme.subtitle2!.copyWith(
           fontWeight: FontWeight.w300,
         );
diff --git a/lib/common/ui/viewmodels/root_view_model.dart b/lib/common/ui/viewmodels/root_view_model.dart
index dd2a360a..f976bcb2 100644
--- a/lib/common/ui/viewmodels/root_view_model.dart
+++ b/lib/common/ui/viewmodels/root_view_model.dart
@@ -5,25 +5,25 @@ import 'package:dhbwstudentapp/common/ui/viewmodels/base_view_model.dart';
 class RootViewModel extends BaseViewModel {
   final PreferencesProvider _preferencesProvider;
 
-  AppTheme _appTheme;
+  late AppTheme _appTheme;
   AppTheme get appTheme => _appTheme;
 
-  bool _isOnboarding;
+  late bool _isOnboarding;
   bool get isOnboarding => _isOnboarding;
 
   RootViewModel(this._preferencesProvider);
 
   Future<void> loadFromPreferences() async {
-    var darkMode = await _preferencesProvider.appTheme();
-
-    _appTheme = darkMode;
+    _appTheme = await _preferencesProvider.appTheme();
     _isOnboarding = await _preferencesProvider.isFirstStart();
 
     notifyListeners("appTheme");
     notifyListeners("isOnboarding");
   }
 
-  Future<void> setAppTheme(AppTheme value) async {
+  Future<void> setAppTheme(AppTheme? value) async {
+    if (value == null) return;
+
     await _preferencesProvider.setAppTheme(value);
     _appTheme = value;
     notifyListeners("appTheme");
diff --git a/lib/common/ui/widgets/dots_indicator.dart b/lib/common/ui/widgets/dots_indicator.dart
index 816e3c79..abcf6f5f 100644
--- a/lib/common/ui/widgets/dots_indicator.dart
+++ b/lib/common/ui/widgets/dots_indicator.dart
@@ -4,7 +4,8 @@ class DotsIndicator extends StatelessWidget {
   final int numberSteps;
   final int currentStep;
 
-  const DotsIndicator({Key key, this.numberSteps, this.currentStep})
+  const DotsIndicator(
+      {Key? key, required this.numberSteps, required this.currentStep})
       : super(key: key);
 
   @override
diff --git a/lib/common/ui/widgets/error_display.dart b/lib/common/ui/widgets/error_display.dart
index 6cefef8f..6be52a66 100644
--- a/lib/common/ui/widgets/error_display.dart
+++ b/lib/common/ui/widgets/error_display.dart
@@ -5,7 +5,7 @@ import 'package:flutter/material.dart';
 class ErrorDisplay extends StatelessWidget {
   final bool show;
 
-  const ErrorDisplay({Key key, this.show}) : super(key: key);
+  const ErrorDisplay({Key? key, required this.show}) : super(key: key);
 
   @override
   Widget build(BuildContext context) {
@@ -24,7 +24,7 @@ class ErrorDisplay extends StatelessWidget {
                       child: Text(
                         L.of(context).noConnectionMessage,
                         textAlign: TextAlign.center,
-                        style: Theme.of(context).textTheme.subtitle2.copyWith(
+                        style: Theme.of(context).textTheme.subtitle2!.copyWith(
                               color: colorNoConnectionForeground(),
                             ),
                       ),
diff --git a/lib/common/ui/widgets/title_list_tile.dart b/lib/common/ui/widgets/title_list_tile.dart
index c3576a63..82de6d27 100644
--- a/lib/common/ui/widgets/title_list_tile.dart
+++ b/lib/common/ui/widgets/title_list_tile.dart
@@ -3,7 +3,7 @@ import 'package:flutter/material.dart';
 class TitleListTile extends StatelessWidget {
   final String title;
 
-  const TitleListTile({Key key, this.title}) : super(key: key);
+  const TitleListTile({Key? key, required this.title}) : super(key: key);
 
   @override
   Widget build(BuildContext context) {
diff --git a/lib/common/util/cancelable_mutex.dart b/lib/common/util/cancelable_mutex.dart
index 5335981a..3160b39c 100644
--- a/lib/common/util/cancelable_mutex.dart
+++ b/lib/common/util/cancelable_mutex.dart
@@ -3,8 +3,8 @@ import 'cancellation_token.dart';
 
 class CancelableMutex {
   final Mutex _mutex = Mutex();
-  CancellationToken _token;
-  CancellationToken get token => _token;
+  CancellationToken? _token;
+  CancellationToken? get token => _token;
 
   void cancel() {
     _token?.cancel();
diff --git a/lib/common/util/cancellation_token.dart b/lib/common/util/cancellation_token.dart
index 59f097cc..36c5e604 100644
--- a/lib/common/util/cancellation_token.dart
+++ b/lib/common/util/cancellation_token.dart
@@ -2,7 +2,7 @@ typedef CancellationCallback = void Function();
 
 class CancellationToken {
   bool _isCancelled = false;
-  CancellationCallback _callback;
+  CancellationCallback? _callback;
 
   bool isCancelled() {
     return _isCancelled;
@@ -17,10 +17,10 @@ class CancellationToken {
   void cancel() {
     _isCancelled = true;
 
-    if (_callback != null) _callback();
+    _callback?.call();
   }
 
-  void setCancellationCallback(CancellationCallback callback) {
+  void setCancellationCallback(CancellationCallback? callback) {
     _callback = callback;
   }
 }
diff --git a/lib/common/util/date_utils.dart b/lib/common/util/date_utils.dart
index 57b54605..19903804 100644
--- a/lib/common/util/date_utils.dart
+++ b/lib/common/util/date_utils.dart
@@ -1,20 +1,22 @@
-DateTime toStartOfDay(DateTime dateTime) {
+// TODO: [Leptopoda] write extension methods directly on [DateTime]
+
+DateTime? toStartOfDay(DateTime? dateTime) {
   return dateTime == null
       ? null
       : DateTime(dateTime.year, dateTime.month, dateTime.day, 0, 0, 0);
 }
 
-DateTime toStartOfMonth(DateTime dateTime) {
+DateTime? toStartOfMonth(DateTime? dateTime) {
   return dateTime == null
       ? null
       : DateTime(dateTime.year, dateTime.month, 1, 0, 0, 0);
 }
 
-DateTime tomorrow(DateTime dateTime) {
+DateTime? tomorrow(DateTime? dateTime) {
   return addDays(dateTime, 1);
 }
 
-DateTime addDays(DateTime dateTime, int days) {
+DateTime? addDays(DateTime? dateTime, int days) {
   return dateTime == null
       ? null
       : DateTime(
@@ -28,12 +30,10 @@ DateTime addDays(DateTime dateTime, int days) {
 }
 
 DateTime toMonday(DateTime dateTime) {
-  return dateTime == null
-      ? null
-      : dateTime.subtract(Duration(days: dateTime.weekday - 1));
+  return dateTime.subtract(Duration(days: dateTime.weekday - 1));
 }
 
-DateTime toPreviousWeek(DateTime dateTime) {
+DateTime? toPreviousWeek(DateTime? dateTime) {
   return dateTime == null
       ? null
       : DateTime(
@@ -46,7 +46,7 @@ DateTime toPreviousWeek(DateTime dateTime) {
         );
 }
 
-DateTime toNextWeek(DateTime dateTime) {
+DateTime? toNextWeek(DateTime? dateTime) {
   return dateTime == null
       ? null
       : DateTime(
@@ -60,20 +60,17 @@ DateTime toNextWeek(DateTime dateTime) {
 }
 
 DateTime toNextMonth(DateTime dateTime) {
-  return dateTime == null
-      ? null
-      : DateTime(
-          dateTime.year,
-          dateTime.month + 1,
-          dateTime.day,
-          dateTime.hour,
-          dateTime.minute,
-          dateTime.second,
-        );
+  return DateTime(
+    dateTime.year,
+    dateTime.month + 1,
+    dateTime.day,
+    dateTime.hour,
+    dateTime.minute,
+    dateTime.second,
+  );
 }
 
 DateTime toTimeOfDay(DateTime dateTime, int hour, int minute) {
-  if (dateTime == null) return null;
   return DateTime(
     dateTime.year,
     dateTime.month,
@@ -84,8 +81,6 @@ DateTime toTimeOfDay(DateTime dateTime, int hour, int minute) {
 }
 
 DateTime toTimeOfDayInFuture(DateTime dateTime, int hour, int minute) {
-  if (dateTime == null) return null;
-
   var newDateTime = DateTime(
     dateTime.year,
     dateTime.month,
@@ -94,7 +89,7 @@ DateTime toTimeOfDayInFuture(DateTime dateTime, int hour, int minute) {
     minute,
   );
 
-  if (dateTime.isAfter(newDateTime)) newDateTime = tomorrow(newDateTime);
+  if (dateTime.isAfter(newDateTime)) newDateTime = tomorrow(newDateTime)!;
 
   return newDateTime;
 }
@@ -105,7 +100,7 @@ bool isAtSameDay(DateTime date1, DateTime date2) {
       date1.day == date2.day;
 }
 
-DateTime toDayOfWeek(DateTime dateTime, int weekday) {
+DateTime? toDayOfWeek(DateTime? dateTime, int weekday) {
   if (dateTime == null) return null;
 
   var startOfWeek = addDays(dateTime, -dateTime.weekday);
diff --git a/lib/common/util/platform_util.dart b/lib/common/util/platform_util.dart
index a602f6c6..a015d709 100644
--- a/lib/common/util/platform_util.dart
+++ b/lib/common/util/platform_util.dart
@@ -19,7 +19,7 @@ class PlatformUtil {
 
   static Brightness platformBrightness() {
     final data = MediaQueryData.fromWindow(WidgetsBinding.instance.window);
-    return data.platformBrightness ?? Brightness.light;
+    return data.platformBrightness;
   }
 
   static Future<void> initializePortraitLandscapeMode() async {
diff --git a/lib/common/util/string_utils.dart b/lib/common/util/string_utils.dart
index f308d63b..d910d0e5 100644
--- a/lib/common/util/string_utils.dart
+++ b/lib/common/util/string_utils.dart
@@ -1,7 +1,9 @@
+// TODO: [Leptopoda] deprecate as dart has it's own function (at least for the above)
+
 String concatStringList(List<String> list, String separator) {
   var result = "";
 
-  for (var element in list ?? []) {
+  for (var element in list) {
     result += element + separator;
   }
 
@@ -12,10 +14,10 @@ String concatStringList(List<String> list, String separator) {
   return result;
 }
 
-String interpolate(String string, List<String> params) {
+String interpolate(String string, List<String?> params) {
   String result = string;
   for (int i = 0; i < params.length; i++) {
-    result = result.replaceAll('%$i', params[i]);
+    result = result.replaceAll('%$i', params[i]!);
   }
 
   return result;
diff --git a/lib/date_management/business/date_entry_provider.dart b/lib/date_management/business/date_entry_provider.dart
index 714d453d..05807f28 100644
--- a/lib/date_management/business/date_entry_provider.dart
+++ b/lib/date_management/business/date_entry_provider.dart
@@ -13,16 +13,16 @@ class DateEntryProvider {
   Future<List<DateEntry>> getCachedDateEntries(
     DateSearchParameters parameters,
   ) async {
-    var cachedEntries = <DateEntry>[];
+    List<DateEntry> cachedEntries = <DateEntry>[];
 
-    if (parameters.includeFuture && parameters.includePast) {
+    if (parameters.includeFuture! && parameters.includePast!) {
       cachedEntries = await _dateEntryRepository.queryAllDateEntries(
         parameters.databaseName,
         parameters.year,
       );
     } else {
       var now = DateTime.now();
-      if (parameters.includeFuture) {
+      if (parameters.includeFuture!) {
         var datesAfter = await _dateEntryRepository.queryDateEntriesAfter(
           parameters.databaseName,
           parameters.year,
@@ -31,7 +31,7 @@ class DateEntryProvider {
         cachedEntries.addAll(datesAfter);
       }
 
-      if (parameters.includePast) {
+      if (parameters.includePast!) {
         var datesBefore = await _dateEntryRepository.queryDateEntriesBefore(
           parameters.databaseName,
           parameters.year,
@@ -41,8 +41,9 @@ class DateEntryProvider {
       }
     }
 
-    cachedEntries.sort((DateEntry a1, DateEntry a2) =>
-        a1?.start?.compareTo(a2?.start));
+    cachedEntries.sort(
+      (DateEntry a1, DateEntry a2) => a1.start.compareTo(a2.start),
+    );
 
     print("Read cached ${cachedEntries.length} date entries");
 
@@ -51,7 +52,7 @@ class DateEntryProvider {
 
   Future<List<DateEntry>> getDateEntries(
     DateSearchParameters parameters,
-    CancellationToken cancellationToken,
+    CancellationToken? cancellationToken,
   ) async {
     var updatedEntries = await _dateEntryService.queryAllDates(
       parameters,
@@ -82,11 +83,11 @@ class DateEntryProvider {
         continue;
       }
 
-      if (dateEntry.start.isBefore(now) && !parameters.includePast) {
+      if (dateEntry.start.isBefore(now) && !parameters.includePast!) {
         continue;
       }
 
-      if (dateEntry.end.isAfter(now) && !parameters.includeFuture) {
+      if (dateEntry.end.isAfter(now) && !parameters.includeFuture!) {
         continue;
       }
 
diff --git a/lib/date_management/data/calendar_access.dart b/lib/date_management/data/calendar_access.dart
index d57bdec7..cf6c852c 100644
--- a/lib/date_management/data/calendar_access.dart
+++ b/lib/date_management/data/calendar_access.dart
@@ -5,8 +5,6 @@ import 'package:dhbwstudentapp/date_management/model/date_entry.dart';
 import 'package:flutter/services.dart';
 import 'package:timezone/timezone.dart' as tz;
 
-
-
 enum CalendarPermission {
   PermissionGranted,
   PermissionDenied,
@@ -19,14 +17,13 @@ enum CalendarPermission {
 class CalendarAccess {
   final DeviceCalendarPlugin _deviceCalendarPlugin = DeviceCalendarPlugin();
 
-
   Future<CalendarPermission> requestCalendarPermission() async {
     try {
       var permissionsGranted = await _deviceCalendarPlugin.hasPermissions();
 
-      if (permissionsGranted.isSuccess && !permissionsGranted.data) {
+      if (permissionsGranted.isSuccess && !permissionsGranted.data!) {
         permissionsGranted = await _deviceCalendarPlugin.requestPermissions();
-        if (!permissionsGranted.isSuccess || !permissionsGranted.data) {
+        if (!permissionsGranted.isSuccess || !permissionsGranted.data!) {
           return CalendarPermission.PermissionDenied;
         }
       }
@@ -42,8 +39,8 @@ class CalendarAccess {
   Future<List<Calendar>> queryWriteableCalendars() async {
     final calendarsResult = await _deviceCalendarPlugin.retrieveCalendars();
     var writeableCalendars = <Calendar>[];
-    for (var calendar in calendarsResult?.data ?? <Calendar>[]) {
-      if (!calendar.isReadOnly) {
+    for (var calendar in calendarsResult.data ?? []) {
+      if (!calendar.isReadOnly!) {
         writeableCalendars.add(calendar);
       }
     }
@@ -53,12 +50,12 @@ class CalendarAccess {
 
   Future<void> addOrUpdateDates(
     List<DateEntry> dateEntries,
-    Calendar calendar,
+    Calendar? calendar,
   ) async {
-    if ((dateEntries ?? []).isEmpty) return;
+    if (dateEntries.isEmpty) return;
 
     var existingEvents =
-        await _getExistingEventsFromCalendar(dateEntries, calendar);
+        await _getExistingEventsFromCalendar(dateEntries, calendar!);
 
     for (var entry in dateEntries) {
       await _addOrUpdateEntry(existingEvents, entry, calendar);
@@ -82,7 +79,6 @@ class CalendarAccess {
       end = entry.end;
     }
 
-
     return await _deviceCalendarPlugin.createOrUpdateEvent(Event(
       calendar.id,
       location: entry.room,
@@ -95,11 +91,13 @@ class CalendarAccess {
     ));
   }
 
-  String _getIdOfExistingEvent(List<Event> existingEvents, DateEntry entry) {
+  String? _getIdOfExistingEvent(List<Event> existingEvents, DateEntry entry) {
     var existingEvent = existingEvents
-        .where((element) => (element.title == entry.description && element.start.toUtc().isAtSameMomentAs(entry.start.toUtc())))
+        .where((element) => (element.title == entry.description &&
+            (element.start?.toUtc().isAtSameMomentAs(entry.start.toUtc()) ??
+                false)))
         .toList();
-    String id;
+    String? id;
 
     if (existingEvent.isNotEmpty) {
       id = existingEvent[0].eventId;
@@ -122,7 +120,7 @@ class CalendarAccess {
     var existingEvents = <Event>[];
 
     if (existingEventsResult.isSuccess) {
-      existingEvents = existingEventsResult.data.toList();
+      existingEvents = existingEventsResult.data!.toList();
     }
     return existingEvents;
   }
@@ -131,7 +129,7 @@ class CalendarAccess {
     var firstEntry = entries[0];
 
     for (var entry in entries) {
-      if (entry.end.isBefore(firstEntry?.end)) {
+      if (entry.end.isBefore(firstEntry.end)) {
         firstEntry = entry;
       }
     }
diff --git a/lib/date_management/data/date_entry_entity.dart b/lib/date_management/data/date_entry_entity.dart
index 20302338..b3b23a75 100644
--- a/lib/date_management/data/date_entry_entity.dart
+++ b/lib/date_management/data/date_entry_entity.dart
@@ -2,22 +2,17 @@ import 'package:dhbwstudentapp/common/data/database_entity.dart';
 import 'package:dhbwstudentapp/date_management/model/date_entry.dart';
 
 class DateEntryEntity extends DatabaseEntity {
-  DateEntry _dateEntry;
+  // TODO: [Leptopoda] make non null
+  DateEntry? _dateEntry;
 
-  DateEntryEntity.fromModel(DateEntry dateEntry) {
-    _dateEntry = dateEntry;
-  }
+  DateEntryEntity.fromModel(DateEntry dateEntry) : _dateEntry = dateEntry;
 
   DateEntryEntity.fromMap(Map<String, dynamic> map) {
     fromMap(map);
   }
 
-  @override
   void fromMap(Map<String, dynamic> map) {
-    DateTime date;
-    if (map["date"] != null) {
-      date = DateTime.fromMillisecondsSinceEpoch(map["date"]);
-    }
+    var date = DateTime.fromMillisecondsSinceEpoch(map["date"]);
 
     _dateEntry = DateEntry(
       comment: map["comment"],
@@ -26,22 +21,24 @@ class DateEntryEntity extends DatabaseEntity {
       databaseName: map["databaseName"],
       start: date,
       end: date,
-      room: map["room"]
+      room: map["room"],
     );
   }
 
   @override
   Map<String, dynamic> toMap() {
+    assert(_dateEntry != null);
     return {
-      "date": _dateEntry.start?.millisecondsSinceEpoch ?? 0,
-      "comment": _dateEntry.comment ?? "",
-      "description": _dateEntry.description ?? "",
-      "year": _dateEntry.year ?? "",
-      "databaseName": _dateEntry.databaseName ?? ""
+      "date": _dateEntry!.start.millisecondsSinceEpoch,
+      "comment": _dateEntry!.comment,
+      "description": _dateEntry!.description,
+      "year": _dateEntry!.year,
+      "databaseName": _dateEntry!.databaseName,
     };
   }
 
-  DateEntry asDateEntry() => _dateEntry;
+  DateEntry asDateEntry() => _dateEntry!;
 
+  // TODO: [Leptopoda] make getter or even constant.
   static String tableName() => "DateEntries";
 }
diff --git a/lib/date_management/data/date_entry_repository.dart b/lib/date_management/data/date_entry_repository.dart
index 7a4f675b..7fc21c7b 100644
--- a/lib/date_management/data/date_entry_repository.dart
+++ b/lib/date_management/data/date_entry_repository.dart
@@ -8,8 +8,8 @@ class DateEntryRepository {
   DateEntryRepository(this._database);
 
   Future<List<DateEntry>> queryAllDateEntries(
-    String databaseName,
-    String year,
+    String? databaseName,
+    String? year,
   ) async {
     var rows = await _database.queryRows(
       DateEntryEntity.tableName(),
@@ -20,7 +20,7 @@ class DateEntryRepository {
     return _rowsToDateEntries(rows);
   }
 
-  Future<List<DateEntry>> queryDateEntriesBetween(
+  Future<List<DateEntry?>> queryDateEntriesBetween(
     String databaseName,
     String year,
     DateTime start,
@@ -41,8 +41,8 @@ class DateEntryRepository {
   }
 
   Future<List<DateEntry>> queryDateEntriesAfter(
-    String databaseName,
-    String year,
+    String? databaseName,
+    String? year,
     DateTime date,
   ) async {
     var rows = await _database.queryRows(
@@ -59,8 +59,8 @@ class DateEntryRepository {
   }
 
   Future<List<DateEntry>> queryDateEntriesBefore(
-    String databaseName,
-    String year,
+    String? databaseName,
+    String? year,
     DateTime date,
   ) async {
     var rows = await _database.queryRows(
@@ -88,8 +88,8 @@ class DateEntryRepository {
   }
 
   Future deleteAllDateEntries(
-    String databaseName,
-    String year,
+    String? databaseName,
+    String? year,
   ) async {
     await _database.deleteWhere(DateEntryEntity.tableName(),
         where: "databaseName=? AND year=?",
diff --git a/lib/date_management/model/date_entry.dart b/lib/date_management/model/date_entry.dart
index 02ba7271..17e4ce98 100644
--- a/lib/date_management/model/date_entry.dart
+++ b/lib/date_management/model/date_entry.dart
@@ -5,15 +5,20 @@ class DateEntry {
   final String databaseName;
   final DateTime start;
   final DateTime end;
-  final String room;
+  final String? room;
 
   DateEntry({
-    this.description,
-    this.year,
-    this.comment,
-    this.databaseName,
-    this.start,
-    this.end,
-    this.room
-  });
+    String? description,
+    String? year,
+    String? comment,
+    String? databaseName,
+    DateTime? start,
+    DateTime? end,
+    this.room,
+  })  : this.start = start ?? DateTime.fromMicrosecondsSinceEpoch(0),
+        this.end = end ?? DateTime.fromMicrosecondsSinceEpoch(0),
+        this.comment = comment ?? "",
+        this.description = description ?? "",
+        this.year = year ?? "",
+        this.databaseName = databaseName ?? "";
 }
diff --git a/lib/date_management/model/date_search_parameters.dart b/lib/date_management/model/date_search_parameters.dart
index a6a1dfc5..f686fc9f 100644
--- a/lib/date_management/model/date_search_parameters.dart
+++ b/lib/date_management/model/date_search_parameters.dart
@@ -1,8 +1,8 @@
 class DateSearchParameters {
-  bool includePast;
-  bool includeFuture;
-  String year;
-  String databaseName;
+  bool? includePast;
+  bool? includeFuture;
+  String? year;
+  String? databaseName;
 
   DateSearchParameters(
     this.includePast,
diff --git a/lib/date_management/service/date_management_service.dart b/lib/date_management/service/date_management_service.dart
index a192677a..0e5d11fa 100644
--- a/lib/date_management/service/date_management_service.dart
+++ b/lib/date_management/service/date_management_service.dart
@@ -7,7 +7,7 @@ import 'package:dhbwstudentapp/dualis/service/session.dart';
 class DateManagementService {
   Future<List<DateEntry>> queryAllDates(
     DateSearchParameters parameters,
-    CancellationToken cancellationToken,
+    CancellationToken? cancellationToken,
   ) async {
     var queryResult = await Session().get(
       _buildRequestUrl(parameters),
@@ -28,7 +28,7 @@ class DateManagementService {
     url += "&sel_typ=pub";
     url += "&sel_jahrgang= ${parameters.year ?? "*"}";
     url += "&sel_bezeichnung=*";
-    url += "&selection=${parameters.includePast ? "**" : "*"}";
+    url += "&selection=${parameters.includePast! ? "**" : "*"}";
     url += "&permission=show";
     url += "&sessionid=";
     url += "&user=nobody";
diff --git a/lib/date_management/service/parsing/all_dates_extract.dart b/lib/date_management/service/parsing/all_dates_extract.dart
index 3db64a4f..828c75bb 100644
--- a/lib/date_management/service/parsing/all_dates_extract.dart
+++ b/lib/date_management/service/parsing/all_dates_extract.dart
@@ -5,7 +5,8 @@ import 'package:intl/intl.dart';
 // TODO: Parse exception to common module
 
 class AllDatesExtract {
-  List<DateEntry> extractAllDates(String body, String databaseName) {
+  List<DateEntry> extractAllDates(String? body, String? databaseName) {
+    if (body == null) return [];
     try {
       return _extractAllDates(body, databaseName);
     } catch (e, trace) {
@@ -14,7 +15,7 @@ class AllDatesExtract {
     }
   }
 
-  List<DateEntry> _extractAllDates(String body, String databaseName) {
+  List<DateEntry> _extractAllDates(String body, String? databaseName) {
     body = body.replaceAll(new RegExp("<(br|BR)[ /]*>"), "\n");
     var document = parse(body);
 
@@ -24,7 +25,7 @@ class AllDatesExtract {
     var dateEntries = <DateEntry>[];
 
     for (var a in dateContainingElement.nodes.sublist(0)) {
-      var text = a.text;
+      var text = a.text!;
 
       var lines = text.split("\n");
 
@@ -38,12 +39,12 @@ class AllDatesExtract {
     }
 
     dateEntries
-        .sort((DateEntry e1, DateEntry e2) => e1.start?.compareTo(e2.start));
+        .sort((DateEntry e1, DateEntry e2) => e1.start.compareTo(e2.start));
 
     return dateEntries;
   }
 
-  DateEntry _parseDateEntryLine(String line, String databaseName) {
+  DateEntry? _parseDateEntryLine(String line, String? databaseName) {
     var parts = line.split(';');
 
     if (parts.length != 5) {
@@ -64,7 +65,7 @@ class AllDatesExtract {
         end: date);
   }
 
-  DateTime _parseDateTime(String date, String time) {
+  DateTime? _parseDateTime(String date, String time) {
     if (time == "24:00") {
       time = "00:00";
     }
diff --git a/lib/date_management/ui/calendar_export_page.dart b/lib/date_management/ui/calendar_export_page.dart
index 6ee2188d..9a439d32 100644
--- a/lib/date_management/ui/calendar_export_page.dart
+++ b/lib/date_management/ui/calendar_export_page.dart
@@ -15,32 +15,25 @@ class CalendarExportPage extends StatefulWidget {
   final bool isCalendarSyncEnabled;
 
   const CalendarExportPage(
-      {Key key,
-      this.entriesToExport,
+      {Key? key,
+      required this.entriesToExport,
       this.isCalendarSyncWidget = false,
       this.isCalendarSyncEnabled = false})
       : super(key: key);
 
   @override
-  _CalendarExportPageState createState() => _CalendarExportPageState(
-      entriesToExport, isCalendarSyncWidget, isCalendarSyncEnabled);
+  _CalendarExportPageState createState() => _CalendarExportPageState();
 }
 
 class _CalendarExportPageState extends State<CalendarExportPage> {
-  final List<DateEntry> entriesToExport;
-  final bool isCalendarSyncWidget;
-  final bool isCalendarSyncEnabled;
-  CalendarExportViewModel viewModel;
-
-  _CalendarExportPageState(this.entriesToExport, this.isCalendarSyncWidget,
-      this.isCalendarSyncEnabled);
+  late CalendarExportViewModel viewModel;
 
   @override
   void initState() {
     super.initState();
 
-    viewModel = CalendarExportViewModel(entriesToExport, CalendarAccess(),
-        KiwiContainer().resolve<PreferencesProvider>());
+    viewModel = CalendarExportViewModel(widget.entriesToExport,
+        CalendarAccess(), KiwiContainer().resolve<PreferencesProvider>());
     viewModel.setOnPermissionDeniedCallback(() {
       Navigator.of(context).pop();
     });
@@ -57,7 +50,7 @@ class _CalendarExportPageState extends State<CalendarExportPage> {
           actionsIconTheme: Theme.of(context).iconTheme,
           elevation: 0,
           iconTheme: Theme.of(context).iconTheme,
-          title: Text(this.isCalendarSyncWidget
+          title: Text(widget.isCalendarSyncWidget
               ? L.of(context).calendarSyncPageTitle
               : L.of(context).dateManagementExportToCalendar),
           toolbarTextStyle: Theme.of(context).textTheme.bodyText2,
@@ -69,7 +62,7 @@ class _CalendarExportPageState extends State<CalendarExportPage> {
               padding: const EdgeInsets.all(24),
               child: Align(
                 alignment: Alignment.centerLeft,
-                child: Text(this.isCalendarSyncWidget
+                child: Text(widget.isCalendarSyncWidget
                     ? L.of(context).calendarSyncPageSubtitle
                     : L.of(context).dateManagementExportToCalendarDescription),
               ),
@@ -89,12 +82,13 @@ class _CalendarExportPageState extends State<CalendarExportPage> {
   Widget _buildCalendarList() {
     return Expanded(
       child: PropertyChangeConsumer<CalendarExportViewModel, String>(
-        builder: (BuildContext context, CalendarExportViewModel viewModel, _) =>
-            ListView.builder(
-          itemCount: viewModel.calendars.length,
+        builder:
+            (BuildContext context, CalendarExportViewModel? viewModel, _) =>
+                ListView.builder(
+          itemCount: viewModel!.calendars.length,
           itemBuilder: (BuildContext context, int index) {
-            var isSelected = viewModel.selectedCalendar?.id ==
-                viewModel.calendars[index]?.id;
+            var isSelected =
+                viewModel.selectedCalendar?.id == viewModel.calendars[index].id;
 
             return _buildCalendarListEntry(
               viewModel.calendars[index],
@@ -109,21 +103,21 @@ class _CalendarExportPageState extends State<CalendarExportPage> {
   Widget _buildStopCalendarSyncBtn() {
     // Dont display the "Synchronisation beenden" button,
     //if synchronization is not enabled or if it is not the right page
-    if (!this.isCalendarSyncWidget) return SizedBox();
+    if (!widget.isCalendarSyncWidget) return SizedBox();
 
     return PropertyChangeProvider<CalendarExportViewModel, String>(
       value: viewModel,
       child: Column(
         children: [
           Container(
-            decoration: this.isCalendarSyncEnabled ? null : null,
+            decoration: widget.isCalendarSyncEnabled ? null : null,
             child: ListTile(
-              enabled: this.isCalendarSyncEnabled ? true : false,
+              enabled: widget.isCalendarSyncEnabled ? true : false,
               title: Text(
                 L.of(context).calendarSyncPageEndSync.toUpperCase(),
                 textAlign: TextAlign.center,
                 style: TextStyle(
-                    color: this.isCalendarSyncEnabled
+                    color: widget.isCalendarSyncEnabled
                         ? ColorPalettes.main
                         : Theme.of(context).disabledColor,
                     fontSize: 14),
@@ -163,9 +157,9 @@ class _CalendarExportPageState extends State<CalendarExportPage> {
               height: 24,
               decoration: BoxDecoration(
                 shape: BoxShape.circle,
-                color: isSelected ? Color(calendar.color) : Colors.transparent,
+                color: isSelected ? Color(calendar.color!) : Colors.transparent,
                 border: Border.all(
-                  color: Color(calendar.color),
+                  color: Color(calendar.color!),
                   width: 4,
                 ),
               ),
@@ -181,7 +175,7 @@ class _CalendarExportPageState extends State<CalendarExportPage> {
             ),
             Padding(
               padding: const EdgeInsets.fromLTRB(16, 0, 0, 0),
-              child: Text(calendar.name),
+              child: Text(calendar.name!),
             ),
           ],
         ),
@@ -191,8 +185,8 @@ class _CalendarExportPageState extends State<CalendarExportPage> {
 
   Widget _buildExportButton() {
     return PropertyChangeConsumer<CalendarExportViewModel, String>(
-      builder: (BuildContext context, CalendarExportViewModel viewModel, _) =>
-          viewModel.isExporting
+      builder: (BuildContext context, CalendarExportViewModel? viewModel, _) =>
+          viewModel!.isExporting
               ? const Padding(
                   padding: EdgeInsets.fromLTRB(8, 8, 8, 15),
                   child: SizedBox(
@@ -214,7 +208,7 @@ class _CalendarExportPageState extends State<CalendarExportPage> {
                         borderRadius: BorderRadius.all(Radius.circular(4.0)),
                       ),
                       title: Text(
-                        this.isCalendarSyncWidget
+                        widget.isCalendarSyncWidget
                             ? L
                                 .of(context)
                                 .calendarSyncPageBeginSync
@@ -231,7 +225,7 @@ class _CalendarExportPageState extends State<CalendarExportPage> {
                       ),
                       onTap: viewModel.canExport
                           ? () async {
-                              if (this.isCalendarSyncWidget) {
+                              if (widget.isCalendarSyncWidget) {
                                 var preferencesProvider = KiwiContainer()
                                     .resolve<PreferencesProvider>();
                                 preferencesProvider.setSelectedCalendar(
diff --git a/lib/date_management/ui/date_management_navigation_entry.dart b/lib/date_management/ui/date_management_navigation_entry.dart
index 57856113..04806a09 100644
--- a/lib/date_management/ui/date_management_navigation_entry.dart
+++ b/lib/date_management/ui/date_management_navigation_entry.dart
@@ -1,5 +1,4 @@
 import 'package:dhbwstudentapp/common/i18n/localizations.dart';
-import 'package:dhbwstudentapp/common/ui/viewmodels/base_view_model.dart';
 import 'package:dhbwstudentapp/date_management/ui/calendar_export_page.dart';
 import 'package:dhbwstudentapp/date_management/ui/date_management_page.dart';
 import 'package:dhbwstudentapp/date_management/ui/viewmodels/date_management_view_model.dart';
@@ -10,13 +9,10 @@ import 'package:flutter/material.dart';
 import 'package:kiwi/kiwi.dart';
 import 'package:property_change_notifier/property_change_notifier.dart';
 
-class DateManagementNavigationEntry extends NavigationEntry {
-  DateManagementViewModel _viewModel;
-
+class DateManagementNavigationEntry
+    extends NavigationEntry<DateManagementViewModel> {
   @override
-  Widget icon(BuildContext context) {
-    return Icon(Icons.date_range);
-  }
+  Icon icon = Icon(Icons.date_range);
 
   @override
   String title(BuildContext context) {
@@ -24,20 +20,15 @@ class DateManagementNavigationEntry extends NavigationEntry {
   }
 
   @override
-  BaseViewModel initViewModel() {
-    if (_viewModel == null) {
-      _viewModel = DateManagementViewModel(
-        KiwiContainer().resolve(),
-        KiwiContainer().resolve(),
-      );
-    }
-
-    return _viewModel;
+  DateManagementViewModel initViewModel() {
+    return DateManagementViewModel(
+      KiwiContainer().resolve(),
+      KiwiContainer().resolve(),
+    );
   }
 
   @override
   List<Widget> appBarActions(BuildContext context) {
-    initViewModel();
     return [
       IconButton(
         icon: Icon(Icons.help_outline),
@@ -47,17 +38,19 @@ class DateManagementNavigationEntry extends NavigationEntry {
         tooltip: L.of(context).helpButtonTooltip,
       ),
       PropertyChangeProvider<DateManagementViewModel, Object>(
-        value: _viewModel,
+        value: model,
         child: PropertyChangeConsumer(
-          builder:
-              (BuildContext context, DateManagementViewModel viewModel, _) =>
-                  PopupMenuButton<String>(
+          builder: (BuildContext context, DateManagementViewModel? _, __) =>
+              PopupMenuButton<String>(
             onSelected: (i) async {
-              await NavigatorKey.rootKey.currentState.push(MaterialPageRoute(
+              await NavigatorKey.rootKey.currentState!.push(
+                MaterialPageRoute(
                   builder: (BuildContext context) => CalendarExportPage(
-                        entriesToExport: viewModel.allDates,
-                      ),
-                  settings: RouteSettings(name: "settings")));
+                    entriesToExport: model.allDates!,
+                  ),
+                  settings: RouteSettings(name: "settings"),
+                ),
+              );
             },
             itemBuilder: (BuildContext context) {
               return [
diff --git a/lib/date_management/ui/date_management_page.dart b/lib/date_management/ui/date_management_page.dart
index 5ace61e4..3e2d124a 100644
--- a/lib/date_management/ui/date_management_page.dart
+++ b/lib/date_management/ui/date_management_page.dart
@@ -1,5 +1,4 @@
 import 'package:dhbwstudentapp/common/i18n/localizations.dart';
-import 'package:dhbwstudentapp/common/ui/viewmodels/base_view_model.dart';
 import 'package:dhbwstudentapp/common/ui/widgets/error_display.dart';
 import 'package:dhbwstudentapp/common/util/date_utils.dart';
 import 'package:dhbwstudentapp/date_management/model/date_entry.dart';
@@ -14,7 +13,8 @@ import 'package:provider/provider.dart';
 class DateManagementPage extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
-    DateManagementViewModel viewModel = Provider.of<BaseViewModel>(context);
+    DateManagementViewModel viewModel =
+        Provider.of<DateManagementViewModel>(context);
 
     return PropertyChangeProvider<DateManagementViewModel, Object>(
       value: viewModel,
@@ -47,17 +47,17 @@ class DateManagementPage extends StatelessWidget {
             child: PropertyChangeConsumer(
               builder: (
                 BuildContext context,
-                DateManagementViewModel model,
+                DateManagementViewModel? model,
                 _,
               ) =>
                   AnimatedSwitcher(
                       duration: const Duration(milliseconds: 200),
                       child: Column(
-                        key: ValueKey(
-                            viewModel?.dateSearchParameters?.toString() ?? ""),
+                        key:
+                            ValueKey(viewModel.dateSearchParameters.toString()),
                         crossAxisAlignment: CrossAxisAlignment.stretch,
                         children: <Widget>[
-                          _buildAllDatesDataTable(model, context),
+                          _buildAllDatesDataTable(model!, context),
                         ],
                       )),
             ),
@@ -94,17 +94,21 @@ class DateManagementPage extends StatelessWidget {
     BuildContext context,
   ) {
     var dataRows = <DataRow>[];
-    for (DateEntry dateEntry in model?.allDates ?? []) {
+    for (DateEntry dateEntry in model.allDates ?? []) {
       dataRows.add(
         DataRow(
           cells: <DataCell>[
             DataCell(
-                Text(dateEntry.description,
-                    style: dateEntry.end.isBefore(DateTime.now())
-                        ? TextStyle(decoration: TextDecoration.lineThrough)
-                        : null), onTap: () {
-              showDateEntryDetailBottomSheet(context, dateEntry);
-            }),
+              Text(
+                dateEntry.description,
+                style: dateEntry.end.isBefore(DateTime.now())
+                    ? TextStyle(decoration: TextDecoration.lineThrough)
+                    : null,
+              ),
+              onTap: () {
+                showDateEntryDetailBottomSheet(context, dateEntry);
+              },
+            ),
             DataCell(
                 Column(
                   mainAxisAlignment: MainAxisAlignment.center,
@@ -138,7 +142,7 @@ class DateManagementPage extends StatelessWidget {
     return dataRows;
   }
 
-  void showDateEntryDetailBottomSheet(BuildContext context, DateEntry entry) {
+  void showDateEntryDetailBottomSheet(BuildContext context, DateEntry? entry) {
     showModalBottomSheet(
       useRootNavigator: true,
       context: context,
@@ -156,10 +160,10 @@ class DateManagementPage extends StatelessWidget {
       properties: const [
         "updateFailed",
       ],
-      builder: (BuildContext context, DateManagementViewModel model,
-              Set properties) =>
+      builder: (BuildContext context, DateManagementViewModel? model,
+              Set? properties) =>
           ErrorDisplay(
-        show: model.updateFailed,
+        show: model!.updateFailed,
       ),
     );
   }
diff --git a/lib/date_management/ui/viewmodels/calendar_export_view_model.dart b/lib/date_management/ui/viewmodels/calendar_export_view_model.dart
index 7bbf6d6a..b4f9a78a 100644
--- a/lib/date_management/ui/viewmodels/calendar_export_view_model.dart
+++ b/lib/date_management/ui/viewmodels/calendar_export_view_model.dart
@@ -11,20 +11,21 @@ class CalendarExportViewModel extends BaseViewModel {
   final PreferencesProvider preferencesProvider;
   final List<DateEntry> _entriesToExport;
 
-  OnPermissionDenied _onPermissionDenied;
+  OnPermissionDenied? _onPermissionDenied;
 
-  List<Calendar> _calendars;
-  List<Calendar> get calendars => _calendars ?? [];
+  List<Calendar>? _calendars;
+  List<Calendar> get calendars => _calendars ??= [];
 
-  Calendar _selectedCalendar;
-  Calendar get selectedCalendar => _selectedCalendar;
+  Calendar? _selectedCalendar;
+  Calendar? get selectedCalendar => _selectedCalendar;
 
   bool get canExport => _selectedCalendar != null;
 
   bool _isExporting = false;
   bool get isExporting => _isExporting;
 
-  CalendarExportViewModel(this._entriesToExport, this.calendarAccess, this.preferencesProvider) {
+  CalendarExportViewModel(
+      this._entriesToExport, this.calendarAccess, this.preferencesProvider) {
     loadCalendars();
   }
 
@@ -41,12 +42,12 @@ class CalendarExportViewModel extends BaseViewModel {
     notifyListeners("_calendars");
   }
 
-  void loadSelectedCalendar() async{
+  void loadSelectedCalendar() async {
     _selectedCalendar = await preferencesProvider.getSelectedCalendar();
     notifyListeners("selectedCalendar");
   }
 
-  void resetSelectedCalendar() async{
+  void resetSelectedCalendar() async {
     await preferencesProvider.setSelectedCalendar(null);
     this.loadCalendars();
   }
diff --git a/lib/date_management/ui/viewmodels/date_management_view_model.dart b/lib/date_management/ui/viewmodels/date_management_view_model.dart
index 2c4b1765..87c1437c 100644
--- a/lib/date_management/ui/viewmodels/date_management_view_model.dart
+++ b/lib/date_management/ui/viewmodels/date_management_view_model.dart
@@ -33,16 +33,16 @@ class DateManagementViewModel extends BaseViewModel {
 
   final CancelableMutex _updateMutex = CancelableMutex();
 
-  Timer _errorResetTimer;
+  Timer? _errorResetTimer;
 
-  List<String> _years;
+  List<String> _years = [];
   List<String> get years => _years;
 
-  String _currentSelectedYear;
-  String get currentSelectedYear => _currentSelectedYear;
+  String? _currentSelectedYear;
+  String? get currentSelectedYear => _currentSelectedYear;
 
-  List<DateEntry> _allDates;
-  List<DateEntry> get allDates => _allDates;
+  List<DateEntry>? _allDates;
+  List<DateEntry>? get allDates => _allDates;
 
   bool _showPassedDates = false;
   bool get showPassedDates => _showPassedDates;
@@ -53,8 +53,8 @@ class DateManagementViewModel extends BaseViewModel {
   bool _isLoading = false;
   bool get isLoading => _isLoading;
 
-  DateDatabase _currentDateDatabase;
-  DateDatabase get currentDateDatabase => _currentDateDatabase;
+  DateDatabase? _currentDateDatabase;
+  DateDatabase? get currentDateDatabase => _currentDateDatabase;
 
   int _dateEntriesKeyIndex = 0;
   int get dateEntriesKeyIndex => _dateEntriesKeyIndex;
@@ -75,8 +75,6 @@ class DateManagementViewModel extends BaseViewModel {
   }
 
   void _buildYearsArray() {
-    _years = [];
-
     for (var i = 2017; i < DateTime.now().year + 3; i++) {
       _years.add(i.toString());
     }
@@ -90,7 +88,8 @@ class DateManagementViewModel extends BaseViewModel {
       notifyListeners("isLoading");
 
       await _doUpdateDates();
-    } catch (_) {} finally {
+    } catch (_) {
+    } finally {
       _isLoading = false;
       _updateMutex.release();
       notifyListeners("isLoading");
@@ -99,11 +98,11 @@ class DateManagementViewModel extends BaseViewModel {
 
   Future<void> _doUpdateDates() async {
     var cachedDateEntries = await _readCachedDateEntries();
-    _updateMutex.token.throwIfCancelled();
+    _updateMutex.token!.throwIfCancelled();
     _setAllDates(cachedDateEntries);
 
     var loadedDateEntries = await _readUpdatedDateEntries();
-    _updateMutex.token.throwIfCancelled();
+    _updateMutex.token!.throwIfCancelled();
 
     if (loadedDateEntries != null) {
       _setAllDates(loadedDateEntries);
@@ -117,23 +116,23 @@ class DateManagementViewModel extends BaseViewModel {
     notifyListeners("updateFailed");
   }
 
-  Future<List<DateEntry>> _readUpdatedDateEntries() async {
+  Future<List<DateEntry>?> _readUpdatedDateEntries() async {
     try {
       var loadedDateEntries = await _dateEntryProvider.getDateEntries(
         dateSearchParameters,
         _updateMutex.token,
       );
       return loadedDateEntries;
-    } on OperationCancelledException {} on ServiceRequestFailed {}
+    } on OperationCancelledException {
+    } on ServiceRequestFailed {}
 
     return null;
   }
 
   Future<List<DateEntry>> _readCachedDateEntries() async {
-    var cachedDateEntries = await _dateEntryProvider.getCachedDateEntries(
+    return _dateEntryProvider.getCachedDateEntries(
       dateSearchParameters,
     );
-    return cachedDateEntries;
   }
 
   void _setAllDates(List<DateEntry> dateEntries) {
@@ -143,24 +142,31 @@ class DateManagementViewModel extends BaseViewModel {
     notifyListeners("allDates");
   }
 
-  void setShowPassedDates(bool value) {
+  // TODO [Leptopoda]: use setters
+  void setShowPassedDates(bool? value) {
+    if (value == null) return;
+
     _showPassedDates = value;
     notifyListeners("showPassedDates");
   }
 
-  void setShowFutureDates(bool value) {
+  void setShowFutureDates(bool? value) {
+    if (value == null) return;
+
     _showFutureDates = value;
     notifyListeners("showFutureDates");
   }
 
-  void setCurrentDateDatabase(DateDatabase database) {
+  void setCurrentDateDatabase(DateDatabase? database) {
     _currentDateDatabase = database;
     notifyListeners("currentDateDatabase");
 
     _preferencesProvider.setLastViewedDateEntryDatabase(database?.id);
   }
 
-  void setCurrentSelectedYear(String year) {
+  void setCurrentSelectedYear(String? year) {
+    if (year == null) return;
+
     _currentSelectedYear = year;
     notifyListeners("currentSelectedYear");
 
@@ -183,19 +189,13 @@ class DateManagementViewModel extends BaseViewModel {
     }
 
     var year = await _preferencesProvider.getLastViewedDateEntryYear();
-    if (year != null) {
-      setCurrentSelectedYear(year);
-    } else {
-      setCurrentSelectedYear(years[0]);
-    }
+    setCurrentSelectedYear(year ?? years[0]);
 
     await updateDates();
   }
 
   void _cancelErrorInFuture() async {
-    if (_errorResetTimer != null) {
-      _errorResetTimer.cancel();
-    }
+    _errorResetTimer?.cancel();
 
     _errorResetTimer = Timer(
       const Duration(seconds: 5),
diff --git a/lib/date_management/ui/widgets/date_detail_bottom_sheet.dart b/lib/date_management/ui/widgets/date_detail_bottom_sheet.dart
index 679eba0e..981ab641 100644
--- a/lib/date_management/ui/widgets/date_detail_bottom_sheet.dart
+++ b/lib/date_management/ui/widgets/date_detail_bottom_sheet.dart
@@ -6,16 +6,16 @@ import 'package:flutter/material.dart';
 import 'package:intl/intl.dart';
 
 class DateDetailBottomSheet extends StatelessWidget {
-  final DateEntry dateEntry;
+  final DateEntry? dateEntry;
 
-  const DateDetailBottomSheet({Key key, this.dateEntry}) : super(key: key);
+  const DateDetailBottomSheet({Key? key, this.dateEntry}) : super(key: key);
 
   @override
   Widget build(BuildContext context) {
     var date = DateFormat.yMd(L.of(context).locale.languageCode)
-        .format(dateEntry.start);
+        .format(dateEntry!.start);
     var time = DateFormat.Hm(L.of(context).locale.languageCode)
-        .format(dateEntry.start);
+        .format(dateEntry!.start);
 
     return Container(
       height: 400,
@@ -46,7 +46,7 @@ class DateDetailBottomSheet extends StatelessWidget {
                 children: <Widget>[
                   Expanded(
                     child: Text(
-                      dateEntry.description,
+                      dateEntry!.description,
                       style: Theme.of(context).textTheme.headline5,
                     ),
                   ),
@@ -60,7 +60,7 @@ class DateDetailBottomSheet extends StatelessWidget {
                           softWrap: true,
                           style: Theme.of(context).textTheme.subtitle2,
                         ),
-                        isAtMidnight(dateEntry.start)
+                        isAtMidnight(dateEntry!.start)
                             ? Container()
                             : Text(
                                 time,
@@ -75,7 +75,7 @@ class DateDetailBottomSheet extends StatelessWidget {
             Padding(
               padding: const EdgeInsets.fromLTRB(0, 8, 0, 0),
               child: Text(
-                dateEntry.comment,
+                dateEntry!.comment,
               ),
             ),
           ],
diff --git a/lib/date_management/ui/widgets/date_filter_options.dart b/lib/date_management/ui/widgets/date_filter_options.dart
index de063007..1eaf161a 100644
--- a/lib/date_management/ui/widgets/date_filter_options.dart
+++ b/lib/date_management/ui/widgets/date_filter_options.dart
@@ -6,19 +6,16 @@ import 'package:flutter/material.dart';
 class DateFilterOptions extends StatefulWidget {
   final DateManagementViewModel viewModel;
 
-  const DateFilterOptions({Key key, this.viewModel}) : super(key: key);
+  const DateFilterOptions({Key? key, required this.viewModel})
+      : super(key: key);
 
   @override
-  _DateFilterOptionsState createState() => _DateFilterOptionsState(viewModel);
+  _DateFilterOptionsState createState() => _DateFilterOptionsState();
 }
 
 class _DateFilterOptionsState extends State<DateFilterOptions> {
-  final DateManagementViewModel viewModel;
-
   bool _isExpanded = false;
 
-  _DateFilterOptionsState(this.viewModel);
-
   @override
   Widget build(BuildContext context) {
     return AnimatedCrossFade(
@@ -81,31 +78,31 @@ class _DateFilterOptionsState extends State<DateFilterOptions> {
   Widget _buildCollapsedChips() {
     var chips = <Widget>[];
 
-    if (viewModel.showPassedDates && viewModel.showFutureDates) {
+    if (widget.viewModel.showPassedDates && widget.viewModel.showFutureDates) {
       chips.add(Chip(
         label: Text(L.of(context).dateManagementChipFutureAndPast),
         visualDensity: VisualDensity.compact,
       ));
-    } else if (viewModel.showFutureDates) {
+    } else if (widget.viewModel.showFutureDates) {
       chips.add(Chip(
         label: Text(L.of(context).dateManagementChipOnlyFuture),
         visualDensity: VisualDensity.compact,
       ));
-    } else if (viewModel.showPassedDates) {
+    } else if (widget.viewModel.showPassedDates) {
       chips.add(Chip(
         label: Text(L.of(context).dateManagementChipOnlyPassed),
         visualDensity: VisualDensity.compact,
       ));
     }
 
-    if (viewModel.currentSelectedYear != null) {
+    if (widget.viewModel.currentSelectedYear != null) {
       chips.add(Chip(
-        label: Text(viewModel.currentSelectedYear),
+        label: Text(widget.viewModel.currentSelectedYear!),
         visualDensity: VisualDensity.compact,
       ));
     }
 
-    var database = viewModel.currentDateDatabase?.displayName ?? "";
+    var database = widget.viewModel.currentDateDatabase?.displayName ?? "";
     if (database != "") {
       chips.add(Chip(
         label: Text(database),
@@ -147,10 +144,8 @@ class _DateFilterOptionsState extends State<DateFilterOptions> {
                     flex: 1,
                     child: DropdownButton(
                       isExpanded: true,
-                      value: viewModel.currentDateDatabase,
-                      onChanged: (value) {
-                        viewModel.setCurrentDateDatabase(value);
-                      },
+                      value: widget.viewModel.currentDateDatabase,
+                      onChanged: widget.viewModel.setCurrentDateDatabase,
                       items: _buildDatabaseMenuItems(),
                     ),
                   ),
@@ -171,10 +166,8 @@ class _DateFilterOptionsState extends State<DateFilterOptions> {
                     flex: 1,
                     child: DropdownButton(
                       isExpanded: true,
-                      value: viewModel.currentSelectedYear,
-                      onChanged: (value) {
-                        viewModel.setCurrentSelectedYear(value);
-                      },
+                      value: widget.viewModel.currentSelectedYear,
+                      onChanged: widget.viewModel.setCurrentSelectedYear,
                       items: _buildYearsMenuItems(),
                     ),
                   ),
@@ -182,19 +175,15 @@ class _DateFilterOptionsState extends State<DateFilterOptions> {
               ),
               CheckboxListTile(
                 title: Text(L.of(context).dateManagementCheckBoxFutureDates),
-                value: viewModel.showFutureDates,
+                value: widget.viewModel.showFutureDates,
                 dense: true,
-                onChanged: (bool value) {
-                  viewModel.setShowFutureDates(value);
-                },
+                onChanged: widget.viewModel.setShowFutureDates,
               ),
               CheckboxListTile(
                 title: Text(L.of(context).dateManagementCheckBoxPassedDates),
-                value: viewModel.showPassedDates,
+                value: widget.viewModel.showPassedDates,
                 dense: true,
-                onChanged: (bool value) {
-                  viewModel.setShowPassedDates(value);
-                },
+                onChanged: widget.viewModel.setShowPassedDates,
               ),
             ],
           ),
@@ -211,7 +200,7 @@ class _DateFilterOptionsState extends State<DateFilterOptions> {
                   _isExpanded = false;
                 });
 
-                viewModel.updateDates();
+                widget.viewModel.updateDates();
               },
             ),
           ),
@@ -223,7 +212,7 @@ class _DateFilterOptionsState extends State<DateFilterOptions> {
   List<DropdownMenuItem<String>> _buildYearsMenuItems() {
     var yearMenuItems = <DropdownMenuItem<String>>[];
 
-    for (var year in viewModel.years) {
+    for (var year in widget.viewModel.years) {
       yearMenuItems.add(DropdownMenuItem(
         child: Text(year),
         value: year,
@@ -235,7 +224,7 @@ class _DateFilterOptionsState extends State<DateFilterOptions> {
   List<DropdownMenuItem<DateDatabase>> _buildDatabaseMenuItems() {
     var databaseMenuItems = <DropdownMenuItem<DateDatabase>>[];
 
-    for (var database in viewModel.allDateDatabases) {
+    for (var database in widget.viewModel.allDateDatabases) {
       databaseMenuItems.add(DropdownMenuItem(
         child: Text(database.displayName),
         value: database,
diff --git a/lib/dualis/model/credentials.dart b/lib/dualis/model/credentials.dart
index 81eb7831..d409e54f 100644
--- a/lib/dualis/model/credentials.dart
+++ b/lib/dualis/model/credentials.dart
@@ -1,10 +1,42 @@
-class Credentials {
-  final String password;
+import 'package:equatable/equatable.dart';
+import 'package:flutter/material.dart';
+
+class Credentials extends Equatable {
   final String username;
+  final String password;
+
+  const Credentials(this.username, this.password);
+
+  @override
+  List<Object?> get props => [username, password];
+}
 
-  Credentials(this.username, this.password);
+class CredentialsEditingController {
+  final _usernameController = TextEditingController();
+  final _passwordController = TextEditingController();
 
-  bool allFieldsFilled() {
-    return password != null && username != null;
+  CredentialsEditingController([Credentials? credentials]) {
+    if (credentials != null) {
+      _usernameController.text = credentials.username;
+      _passwordController.text = credentials.password;
+    }
   }
+
+  set credentials(Credentials credentials) {
+    _usernameController.text = credentials.username;
+    _passwordController.text = credentials.password;
+  }
+
+  void dispose() {
+    _usernameController.dispose();
+    _passwordController.dispose();
+  }
+
+  TextEditingController get username => _usernameController;
+  TextEditingController get password => _passwordController;
+
+  Credentials get credentials => Credentials(
+        _usernameController.text,
+        _passwordController.text,
+      );
 }
diff --git a/lib/dualis/model/exam.dart b/lib/dualis/model/exam.dart
index e8947ddb..22cf8f93 100644
--- a/lib/dualis/model/exam.dart
+++ b/lib/dualis/model/exam.dart
@@ -7,9 +7,9 @@ enum ExamState {
 }
 
 class Exam {
-  final String name;
+  final String? name;
   final ExamGrade grade;
-  final String semester;
+  final String? semester;
   final ExamState state;
 
   Exam(
diff --git a/lib/dualis/model/exam_grade.dart b/lib/dualis/model/exam_grade.dart
index a494edac..0759ddd1 100644
--- a/lib/dualis/model/exam_grade.dart
+++ b/lib/dualis/model/exam_grade.dart
@@ -7,7 +7,7 @@ enum ExamGradeState {
 
 class ExamGrade {
   ExamGradeState state;
-  String gradeValue;
+  String? gradeValue;
 
   ExamGrade.failed()
       : state = ExamGradeState.Failed,
@@ -23,7 +23,7 @@ class ExamGrade {
 
   ExamGrade.graded(this.gradeValue) : state = ExamGradeState.Graded;
 
-  static ExamGrade fromString(String grade) {
+  static ExamGrade fromString(String? grade) {
     if (grade == "noch nicht gesetzt" || grade == "") {
       return ExamGrade.notGraded();
     }
diff --git a/lib/dualis/model/module.dart b/lib/dualis/model/module.dart
index d6e4dc56..c2d1d879 100644
--- a/lib/dualis/model/module.dart
+++ b/lib/dualis/model/module.dart
@@ -2,11 +2,11 @@ import 'package:dhbwstudentapp/dualis/model/exam.dart';
 
 class Module {
   final List<Exam> exams;
-  final String id;
-  final String name;
-  final String credits;
-  final String grade;
-  final ExamState state;
+  final String? id;
+  final String? name;
+  final String? credits;
+  final String? grade;
+  final ExamState? state;
 
   Module(
     this.exams,
diff --git a/lib/dualis/model/semester.dart b/lib/dualis/model/semester.dart
index 84eecc90..db30676b 100644
--- a/lib/dualis/model/semester.dart
+++ b/lib/dualis/model/semester.dart
@@ -1,7 +1,7 @@
 import 'package:dhbwstudentapp/dualis/model/module.dart';
 
 class Semester {
-  final String name;
+  final String? name;
   final List<Module> modules;
 
   Semester(
diff --git a/lib/dualis/model/study_grades.dart b/lib/dualis/model/study_grades.dart
index d57266b2..e5b25ab4 100644
--- a/lib/dualis/model/study_grades.dart
+++ b/lib/dualis/model/study_grades.dart
@@ -1,8 +1,8 @@
 class StudyGrades {
-  final double gpaTotal;
-  final double gpaMainModules;
-  final double creditsTotal;
-  final double creditsGained;
+  final double? gpaTotal;
+  final double? gpaMainModules;
+  final double? creditsTotal;
+  final double? creditsGained;
 
   StudyGrades(
     this.gpaTotal,
diff --git a/lib/dualis/service/cache_dualis_service_decorator.dart b/lib/dualis/service/cache_dualis_service_decorator.dart
index 95a2ec9c..d182de36 100644
--- a/lib/dualis/service/cache_dualis_service_decorator.dart
+++ b/lib/dualis/service/cache_dualis_service_decorator.dart
@@ -1,4 +1,5 @@
 import 'package:dhbwstudentapp/common/util/cancellation_token.dart';
+import 'package:dhbwstudentapp/dualis/model/credentials.dart';
 import 'package:dhbwstudentapp/dualis/model/module.dart';
 import 'package:dhbwstudentapp/dualis/model/semester.dart';
 import 'package:dhbwstudentapp/dualis/model/study_grades.dart';
@@ -10,25 +11,24 @@ import 'package:dhbwstudentapp/dualis/service/dualis_service.dart';
 class CacheDualisServiceDecorator extends DualisService {
   final DualisService _service;
 
-  List<Module> _allModulesCached;
-  List<String> _allSemesterNamesCached;
-  Map<String, Semester> _semestersCached = {};
-  StudyGrades _studyGradesCached;
+  List<Module>? _allModulesCached;
+  List<String>? _allSemesterNamesCached;
+  Map<String?, Semester> _semestersCached = {};
+  StudyGrades? _studyGradesCached;
 
   CacheDualisServiceDecorator(this._service);
 
   @override
   Future<LoginResult> login(
-    String username,
-    String password, [
-    CancellationToken cancellationToken,
+    Credentials credentials, [
+    CancellationToken? cancellationToken,
   ]) {
-    return _service.login(username, password, cancellationToken);
+    return _service.login(credentials, cancellationToken);
   }
 
   @override
   Future<List<Module>> queryAllModules([
-    CancellationToken cancellationToken,
+    CancellationToken? cancellationToken,
   ]) async {
     if (_allModulesCached != null) {
       return Future.value(_allModulesCached);
@@ -43,8 +43,8 @@ class CacheDualisServiceDecorator extends DualisService {
 
   @override
   Future<Semester> querySemester(
-    String name, [
-    CancellationToken cancellationToken,
+    String? name, [
+    CancellationToken? cancellationToken,
   ]) async {
     if (_semestersCached.containsKey(name)) {
       return Future.value(_semestersCached[name]);
@@ -58,7 +58,7 @@ class CacheDualisServiceDecorator extends DualisService {
 
   @override
   Future<List<String>> querySemesterNames([
-    CancellationToken cancellationToken,
+    CancellationToken? cancellationToken,
   ]) async {
     if (_allSemesterNamesCached != null) {
       return Future.value(_allSemesterNamesCached);
@@ -73,7 +73,7 @@ class CacheDualisServiceDecorator extends DualisService {
 
   @override
   Future<StudyGrades> queryStudyGrades([
-    CancellationToken cancellationToken,
+    CancellationToken? cancellationToken,
   ]) async {
     if (_studyGradesCached != null) {
       return Future.value(_studyGradesCached);
@@ -95,7 +95,7 @@ class CacheDualisServiceDecorator extends DualisService {
 
   @override
   Future<void> logout([
-    CancellationToken cancellationToken,
+    CancellationToken? cancellationToken,
   ]) async {
     await _service.logout(cancellationToken);
     clearCache();
diff --git a/lib/dualis/service/dualis_authentication.dart b/lib/dualis/service/dualis_authentication.dart
index 193b0c1b..b7d681ac 100644
--- a/lib/dualis/service/dualis_authentication.dart
+++ b/lib/dualis/service/dualis_authentication.dart
@@ -1,4 +1,5 @@
 import 'package:dhbwstudentapp/common/util/cancellation_token.dart';
+import 'package:dhbwstudentapp/dualis/model/credentials.dart';
 import 'package:dhbwstudentapp/dualis/service/dualis_service.dart';
 import 'package:dhbwstudentapp/dualis/service/dualis_website_model.dart';
 import 'package:dhbwstudentapp/dualis/service/parsing/access_denied_extract.dart';
@@ -16,36 +17,28 @@ import 'package:http/http.dart';
 class DualisAuthentication {
   final RegExp _tokenRegex = RegExp("ARGUMENTS=-N([0-9]{15})");
 
-  String _username;
-  String _password;
+  Credentials? _credentials;
 
-  DualisUrls _dualisUrls;
-  DualisUrls get dualisUrls => _dualisUrls;
+  // TODO: [Leptopoda] make singletons :)
 
-  String _authToken;
-  Session _session;
+  DualisUrls? _dualisUrls;
+  DualisUrls get dualisUrls => _dualisUrls ??= DualisUrls();
 
-  LoginResult _loginState = LoginResult.LoggedOut;
-  LoginResult get loginState => _loginState;
+  String? _authToken;
+  Session? _session;
+  Session get session => _session ??= Session();
+
+  LoginResult? _loginState;
+  LoginResult get loginState => _loginState ??= LoginResult.LoggedOut;
 
   Future<LoginResult> login(
-    String username,
-    String password,
-    CancellationToken cancellationToken,
+    Credentials credentials,
+    CancellationToken? cancellationToken,
   ) async {
-    username = username ?? this._username;
-    password = password ?? this._password;
-
-    _dualisUrls = dualisUrls ?? DualisUrls();
-
-    this._username = username;
-    this._password = password;
-
-    _session = Session();
+    _credentials = credentials;
 
     var loginResponse = await _makeLoginRequest(
-      username,
-      password,
+      credentials,
       cancellationToken,
     );
 
@@ -68,7 +61,7 @@ class DualisAuthentication {
       return loginState;
     }
 
-    var redirectPage = await _session.get(
+    var redirectPage = await session.get(
       redirectUrl,
       cancellationToken,
     );
@@ -83,10 +76,10 @@ class DualisAuthentication {
       return loginState;
     }
 
-    _updateAccessToken(dualisUrls.mainPageUrl);
+    _updateAccessToken(dualisUrls.mainPageUrl!);
 
-    var mainPage = await _session.get(
-      dualisUrls.mainPageUrl,
+    var mainPage = await session.get(
+      dualisUrls.mainPageUrl!,
       cancellationToken,
     );
 
@@ -100,16 +93,15 @@ class DualisAuthentication {
     return loginState;
   }
 
-  Future<Response> _makeLoginRequest(
-    String user,
-    String password, [
-    CancellationToken cancellationToken,
+  Future<Response?> _makeLoginRequest(
+    Credentials credentials, [
+    CancellationToken? cancellationToken,
   ]) async {
     var loginUrl = dualisEndpoint + "/scripts/mgrqispi.dll";
 
     var data = {
-      "usrname": user,
-      "pass": password,
+      "usrname": credentials.username,
+      "pass": credentials.password,
       "APPNAME": "CampusNet",
       "PRGNAME": "LOGINCHECK",
       "ARGUMENTS": "clino,usrname,pass,menuno,menu_type,browser,platform",
@@ -121,7 +113,7 @@ class DualisAuthentication {
     };
 
     try {
-      var loginResponse = await _session.rawPost(
+      var loginResponse = await session.rawPost(
         loginUrl,
         data,
         cancellationToken,
@@ -138,15 +130,19 @@ class DualisAuthentication {
   /// This method handles the authentication cookie and token. If the session
   /// timed out, it will renew the session by logging in again
   ///
-  Future<String> authenticatedGet(
+  Future<String?> authenticatedGet(
     String url,
-    CancellationToken cancellationToken,
+    CancellationToken? cancellationToken,
   ) async {
-    var result = await _session.get(
+    assert(_credentials != null);
+
+    var result = await session.get(
       _fillUrlWithAuthToken(url),
       cancellationToken,
     );
 
+    if (result == null) return null;
+
     cancellationToken?.throwIfCancelled();
 
     if (!TimeoutExtract().isTimeoutErrorPage(result) &&
@@ -154,10 +150,10 @@ class DualisAuthentication {
       return result;
     }
 
-    var loginResult = await login(_username, _password, cancellationToken);
+    var loginResult = await login(_credentials!, cancellationToken);
 
     if (loginResult == LoginResult.LoggedIn) {
-      return await _session.get(
+      return await session.get(
         _fillUrlWithAuthToken(url),
         cancellationToken,
       );
@@ -167,9 +163,9 @@ class DualisAuthentication {
   }
 
   Future<void> logout([
-    CancellationToken cancellationToken,
+    CancellationToken? cancellationToken,
   ]) async {
-    var logoutRequest = _session.get(dualisUrls.logoutUrl, cancellationToken);
+    var logoutRequest = session.get(dualisUrls.logoutUrl, cancellationToken);
 
     _session = null;
     _dualisUrls = null;
@@ -186,9 +182,9 @@ class DualisAuthentication {
   void _updateAccessToken(String urlWithNewToken) {
     var tokenMatch = _tokenRegex.firstMatch(urlWithNewToken);
 
-    if (tokenMatch == null) return;
-
-    _authToken = tokenMatch.group(1);
+    if (tokenMatch != null) {
+      _authToken = tokenMatch.group(1);
+    }
   }
 
   ///
@@ -207,13 +203,13 @@ class DualisAuthentication {
     return url;
   }
 
-  void setLoginCredentials(String username, String password) {
-    _username = username;
-    _password = password;
+  void setLoginCredentials(Credentials credentials) {
+    _credentials = credentials;
   }
 
   Future<LoginResult> loginWithPreviousCredentials(
       CancellationToken cancellationToken) async {
-    return await login(_username, _password, cancellationToken);
+    assert(_credentials != null);
+    return await login(_credentials!, cancellationToken);
   }
 }
diff --git a/lib/dualis/service/dualis_scraper.dart b/lib/dualis/service/dualis_scraper.dart
index 7c9c65fd..b217200b 100644
--- a/lib/dualis/service/dualis_scraper.dart
+++ b/lib/dualis/service/dualis_scraper.dart
@@ -1,4 +1,5 @@
 import 'package:dhbwstudentapp/common/util/cancellation_token.dart';
+import 'package:dhbwstudentapp/dualis/model/credentials.dart';
 import 'package:dhbwstudentapp/dualis/model/study_grades.dart';
 import 'package:dhbwstudentapp/dualis/service/dualis_authentication.dart';
 import 'package:dhbwstudentapp/dualis/service/dualis_service.dart';
@@ -19,10 +20,10 @@ class DualisScraper {
   DualisUrls get _dualisUrls => _dualisAuthentication.dualisUrls;
 
   Future<List<DualisModule>> loadAllModules([
-    CancellationToken cancellationToken,
+    CancellationToken? cancellationToken,
   ]) async {
     var allModulesPageResponse = await _dualisAuthentication.authenticatedGet(
-      _dualisUrls.studentResultsUrl,
+      _dualisUrls.studentResultsUrl!,
       cancellationToken,
     );
 
@@ -31,7 +32,7 @@ class DualisScraper {
 
   Future<List<DualisExam>> loadModuleExams(
     String moduleDetailsUrl, [
-    CancellationToken cancellationToken,
+    CancellationToken? cancellationToken,
   ]) async {
     var detailsResponse = await _dualisAuthentication.authenticatedGet(
       moduleDetailsUrl,
@@ -44,10 +45,10 @@ class DualisScraper {
   }
 
   Future<List<DualisSemester>> loadSemesters([
-    CancellationToken cancellationToken,
+    CancellationToken? cancellationToken,
   ]) async {
     var courseResultsResponse = await _dualisAuthentication.authenticatedGet(
-      _dualisUrls.courseResultUrl,
+      _dualisUrls.courseResultUrl!,
       cancellationToken,
     );
 
@@ -65,12 +66,12 @@ class DualisScraper {
     return semesters;
   }
 
-  Future<List<DualisModule>> loadSemesterModules(
-    String semesterName, [
-    CancellationToken cancellationToken,
+  Future<List<DualisModule?>> loadSemesterModules(
+    String? semesterName, [
+    CancellationToken? cancellationToken,
   ]) async {
     var coursePage = await _dualisAuthentication.authenticatedGet(
-      _dualisUrls.semesterCourseResultUrls[semesterName],
+      _dualisUrls.semesterCourseResultUrls[semesterName!]!,
       cancellationToken,
     );
 
@@ -79,10 +80,10 @@ class DualisScraper {
   }
 
   Future<StudyGrades> loadStudyGrades(
-    CancellationToken cancellationToken,
+    CancellationToken? cancellationToken,
   ) async {
     var studentsResultsPage = await _dualisAuthentication.authenticatedGet(
-      _dualisUrls.studentResultsUrl,
+      _dualisUrls.studentResultsUrl!,
       cancellationToken,
     );
 
@@ -108,11 +109,10 @@ class DualisScraper {
   }
 
   Future<LoginResult> login(
-    String username,
-    String password,
-    CancellationToken cancellationToken,
+    Credentials credentials,
+    CancellationToken? cancellationToken,
   ) {
-    return _dualisAuthentication.login(username, password, cancellationToken);
+    return _dualisAuthentication.login(credentials, cancellationToken);
   }
 
   Future<LoginResult> loginWithPreviousCredentials(
@@ -122,7 +122,7 @@ class DualisScraper {
         .loginWithPreviousCredentials(cancellationToken);
   }
 
-  Future<void> logout(CancellationToken cancellationToken) {
+  Future<void> logout(CancellationToken? cancellationToken) {
     return _dualisAuthentication.logout(cancellationToken);
   }
 
@@ -130,7 +130,7 @@ class DualisScraper {
     return _dualisAuthentication.loginState == LoginResult.LoggedIn;
   }
 
-  void setLoginCredentials(String username, String password) {
-    _dualisAuthentication.setLoginCredentials(username, password);
+  void setLoginCredentials(Credentials credentials) {
+    _dualisAuthentication.setLoginCredentials(credentials);
   }
 }
diff --git a/lib/dualis/service/dualis_service.dart b/lib/dualis/service/dualis_service.dart
index ddbd93fd..a830cbf9 100644
--- a/lib/dualis/service/dualis_service.dart
+++ b/lib/dualis/service/dualis_service.dart
@@ -1,4 +1,5 @@
 import 'package:dhbwstudentapp/common/util/cancellation_token.dart';
+import 'package:dhbwstudentapp/dualis/model/credentials.dart';
 import 'package:dhbwstudentapp/dualis/model/exam.dart';
 import 'package:dhbwstudentapp/dualis/model/module.dart';
 import 'package:dhbwstudentapp/dualis/model/semester.dart';
@@ -7,30 +8,29 @@ import 'package:dhbwstudentapp/dualis/service/dualis_scraper.dart';
 
 abstract class DualisService {
   Future<LoginResult> login(
-    String username,
-    String password, [
-    CancellationToken cancellationToken,
+    Credentials credentials, [
+    CancellationToken? cancellationToken,
   ]);
 
   Future<StudyGrades> queryStudyGrades([
-    CancellationToken cancellationToken,
+    CancellationToken? cancellationToken,
   ]);
 
   Future<List<String>> querySemesterNames([
-    CancellationToken cancellationToken,
+    CancellationToken? cancellationToken,
   ]);
 
   Future<List<Module>> queryAllModules([
-    CancellationToken cancellationToken,
+    CancellationToken? cancellationToken,
   ]);
 
   Future<Semester> querySemester(
-    String name, [
-    CancellationToken cancellationToken,
+    String? name, [
+    CancellationToken? cancellationToken,
   ]);
 
   Future<void> logout([
-    CancellationToken cancellationToken,
+    CancellationToken? cancellationToken,
   ]);
 }
 
@@ -48,27 +48,25 @@ class DualisServiceImpl extends DualisService {
 
   @override
   Future<LoginResult> login(
-    String username,
-    String password, [
-    CancellationToken cancellationToken,
+    Credentials credentials, [
+    CancellationToken? cancellationToken,
   ]) async {
     return await _dualisScraper.login(
-      username,
-      password,
+      credentials,
       cancellationToken,
     );
   }
 
   @override
   Future<StudyGrades> queryStudyGrades([
-    CancellationToken cancellationToken,
+    CancellationToken? cancellationToken,
   ]) async {
     return await _dualisScraper.loadStudyGrades(cancellationToken);
   }
 
   @override
   Future<List<String>> querySemesterNames([
-    CancellationToken cancellationToken,
+    CancellationToken? cancellationToken,
   ]) async {
     var semesters = await _dualisScraper.loadSemesters(cancellationToken);
 
@@ -83,7 +81,7 @@ class DualisServiceImpl extends DualisService {
 
   @override
   Future<List<Module>> queryAllModules([
-    CancellationToken cancellationToken,
+    CancellationToken? cancellationToken,
   ]) async {
     var dualisModules = await _dualisScraper.loadAllModules(cancellationToken);
 
@@ -103,8 +101,8 @@ class DualisServiceImpl extends DualisService {
 
   @override
   Future<Semester> querySemester(
-    String name, [
-    CancellationToken cancellationToken,
+    String? name, [
+    CancellationToken? cancellationToken,
   ]) async {
     var semesterModules = await _dualisScraper.loadSemesterModules(
       name,
@@ -115,7 +113,7 @@ class DualisServiceImpl extends DualisService {
 
     for (var dualisModule in semesterModules) {
       var moduleExams = await _dualisScraper.loadModuleExams(
-        dualisModule.detailsUrl,
+        dualisModule!.detailsUrl!,
         cancellationToken,
       );
 
@@ -145,7 +143,7 @@ class DualisServiceImpl extends DualisService {
 
   @override
   Future<void> logout([
-    CancellationToken cancellationToken,
+    CancellationToken? cancellationToken,
   ]) async {
     await _dualisScraper.logout(cancellationToken);
   }
diff --git a/lib/dualis/service/dualis_website_model.dart b/lib/dualis/service/dualis_website_model.dart
index 8e7bdf60..dc234e45 100644
--- a/lib/dualis/service/dualis_website_model.dart
+++ b/lib/dualis/service/dualis_website_model.dart
@@ -8,18 +8,18 @@ const String dualisEndpoint = "https://dualis.dhbw.de";
 /// access token contained within the urls.
 ///
 class DualisUrls {
-  String courseResultUrl;
-  String studentResultsUrl;
-  String logoutUrl;
-  String mainPageUrl;
-  String monthlyScheduleUrl;
+  String? courseResultUrl;
+  String? studentResultsUrl;
+  late String logoutUrl;
+  String? mainPageUrl;
+  String? monthlyScheduleUrl;
 
-  Map<String, String> semesterCourseResultUrls = {};
+  Map<String, String?> semesterCourseResultUrls = {};
 }
 
 class DualisSemester {
   final String semesterName;
-  final String semesterCourseResultsUrl;
+  final String? semesterCourseResultsUrl;
   final List<DualisModule> modules;
 
   DualisSemester(
@@ -30,12 +30,12 @@ class DualisSemester {
 }
 
 class DualisModule {
-  final String id;
-  final String name;
-  final String finalGrade;
-  final String credits;
-  final String detailsUrl;
-  final ExamState state;
+  final String? id;
+  final String? name;
+  final String? finalGrade;
+  final String? credits;
+  final String? detailsUrl;
+  final ExamState? state;
 
   DualisModule(
     this.id,
@@ -48,11 +48,11 @@ class DualisModule {
 }
 
 class DualisExam {
-  final String tryNr;
-  final String moduleName;
-  final String name;
+  final String? tryNr;
+  final String? moduleName;
+  final String? name;
   final ExamGrade grade;
-  final String semester;
+  final String? semester;
 
   DualisExam(
     this.name,
diff --git a/lib/dualis/service/fake_account_dualis_scraper_decorator.dart b/lib/dualis/service/fake_account_dualis_scraper_decorator.dart
index 54a0e3c0..fddbbce8 100644
--- a/lib/dualis/service/fake_account_dualis_scraper_decorator.dart
+++ b/lib/dualis/service/fake_account_dualis_scraper_decorator.dart
@@ -1,4 +1,5 @@
 import 'package:dhbwstudentapp/common/util/cancellation_token.dart';
+import 'package:dhbwstudentapp/dualis/model/credentials.dart';
 import 'package:dhbwstudentapp/dualis/model/study_grades.dart';
 import 'package:dhbwstudentapp/dualis/service/dualis_scraper.dart';
 import 'package:dhbwstudentapp/dualis/service/dualis_service.dart';
@@ -14,13 +15,12 @@ import 'package:dhbwstudentapp/schedule/model/schedule.dart';
 /// area of the app.
 ///
 class FakeAccountDualisScraperDecorator implements DualisScraper {
-  final fakeUsername = "fakeAccount@domain.de";
-  final fakePassword = "Passw0rd";
+  static const _credentials = Credentials("fakeAccount@domain.de", "Passw0rd");
 
   final DualisScraper _fakeDualisScraper = FakeDataDualisScraper();
   final DualisScraper _originalDualisScraper;
 
-  DualisScraper _currentDualisScraper;
+  late DualisScraper _currentDualisScraper;
 
   FakeAccountDualisScraperDecorator(
     this._originalDualisScraper,
@@ -33,14 +33,14 @@ class FakeAccountDualisScraperDecorator implements DualisScraper {
 
   @override
   Future<List<DualisModule>> loadAllModules(
-      [CancellationToken cancellationToken]) {
+      [CancellationToken? cancellationToken]) {
     return _currentDualisScraper.loadAllModules();
   }
 
   @override
   Future<List<DualisExam>> loadModuleExams(
     String moduleDetailsUrl, [
-    CancellationToken cancellationToken,
+    CancellationToken? cancellationToken,
   ]) {
     return _currentDualisScraper.loadModuleExams(
       moduleDetailsUrl,
@@ -60,9 +60,9 @@ class FakeAccountDualisScraperDecorator implements DualisScraper {
   }
 
   @override
-  Future<List<DualisModule>> loadSemesterModules(
-    String semesterName, [
-    CancellationToken cancellationToken,
+  Future<List<DualisModule?>> loadSemesterModules(
+    String? semesterName, [
+    CancellationToken? cancellationToken,
   ]) {
     return _currentDualisScraper.loadSemesterModules(
       semesterName,
@@ -72,29 +72,27 @@ class FakeAccountDualisScraperDecorator implements DualisScraper {
 
   @override
   Future<List<DualisSemester>> loadSemesters(
-      [CancellationToken cancellationToken]) {
+      [CancellationToken? cancellationToken]) {
     return _currentDualisScraper.loadSemesters(cancellationToken);
   }
 
   @override
-  Future<StudyGrades> loadStudyGrades(CancellationToken cancellationToken) {
+  Future<StudyGrades> loadStudyGrades(CancellationToken? cancellationToken) {
     return _currentDualisScraper.loadStudyGrades(cancellationToken);
   }
 
   @override
   Future<LoginResult> login(
-    String username,
-    String password,
-    CancellationToken cancellationToken,
+    Credentials credentials,
+    CancellationToken? cancellationToken,
   ) {
-    if (username == fakeUsername && password == fakePassword) {
+    if (credentials == _credentials) {
       _currentDualisScraper = _fakeDualisScraper;
     } else {
       _currentDualisScraper = _originalDualisScraper;
     }
     return _currentDualisScraper.login(
-      username,
-      password,
+      credentials,
       cancellationToken,
     );
   }
@@ -108,23 +106,20 @@ class FakeAccountDualisScraperDecorator implements DualisScraper {
   }
 
   @override
-  Future<void> logout(CancellationToken cancellationToken) {
+  Future<void> logout(CancellationToken? cancellationToken) {
     return _currentDualisScraper.logout(
       cancellationToken,
     );
   }
 
   @override
-  void setLoginCredentials(String username, String password) {
-    if (username == fakeUsername && password == fakePassword) {
+  void setLoginCredentials(Credentials credentials) {
+    if (credentials == _credentials) {
       _currentDualisScraper = _fakeDualisScraper;
     } else {
       _currentDualisScraper = _originalDualisScraper;
     }
 
-    return _currentDualisScraper.setLoginCredentials(
-      username,
-      password,
-    );
+    return _currentDualisScraper.setLoginCredentials(credentials);
   }
 }
diff --git a/lib/dualis/service/fake_data_dualis_scraper.dart b/lib/dualis/service/fake_data_dualis_scraper.dart
index fe7a90b0..56de36b8 100644
--- a/lib/dualis/service/fake_data_dualis_scraper.dart
+++ b/lib/dualis/service/fake_data_dualis_scraper.dart
@@ -1,4 +1,5 @@
 import 'package:dhbwstudentapp/common/util/cancellation_token.dart';
+import 'package:dhbwstudentapp/dualis/model/credentials.dart';
 import 'package:dhbwstudentapp/dualis/model/exam.dart';
 import 'package:dhbwstudentapp/dualis/model/exam_grade.dart';
 import 'package:dhbwstudentapp/dualis/model/study_grades.dart';
@@ -20,7 +21,7 @@ class FakeDataDualisScraper implements DualisScraper {
 
   @override
   Future<List<DualisModule>> loadAllModules(
-      [CancellationToken cancellationToken]) async {
+      [CancellationToken? cancellationToken]) async {
     await Future.delayed(Duration(milliseconds: 200));
 
     return Future.value([
@@ -38,7 +39,7 @@ class FakeDataDualisScraper implements DualisScraper {
   @override
   Future<List<DualisExam>> loadModuleExams(
     String moduleDetailsUrl, [
-    CancellationToken cancellationToken,
+    CancellationToken? cancellationToken,
   ]) async {
     await Future.delayed(Duration(milliseconds: 200));
     return Future.value([
@@ -63,8 +64,8 @@ class FakeDataDualisScraper implements DualisScraper {
 
   @override
   Future<List<DualisModule>> loadSemesterModules(
-    String semesterName, [
-    CancellationToken cancellationToken,
+    String? semesterName, [
+    CancellationToken? cancellationToken,
   ]) async {
     await Future.delayed(Duration(milliseconds: 200));
     return Future.value([
@@ -81,14 +82,14 @@ class FakeDataDualisScraper implements DualisScraper {
 
   @override
   Future<List<DualisSemester>> loadSemesters(
-      [CancellationToken cancellationToken]) async {
+      [CancellationToken? cancellationToken]) async {
     await Future.delayed(Duration(milliseconds: 200));
     return Future.value([DualisSemester("SoSe2020", "", [])]);
   }
 
   @override
   Future<StudyGrades> loadStudyGrades(
-      CancellationToken cancellationToken) async {
+      CancellationToken? cancellationToken) async {
     await Future.delayed(Duration(milliseconds: 200));
     return Future.value(
       StudyGrades(
@@ -102,9 +103,8 @@ class FakeDataDualisScraper implements DualisScraper {
 
   @override
   Future<LoginResult> login(
-    String username,
-    String password,
-    CancellationToken cancellationToken,
+    Credentials credentials,
+    CancellationToken? cancellationToken,
   ) async {
     await Future.delayed(Duration(milliseconds: 200));
     _isLoggedIn = true;
@@ -120,13 +120,13 @@ class FakeDataDualisScraper implements DualisScraper {
   }
 
   @override
-  Future<void> logout(CancellationToken cancellationToken) async {
+  Future<void> logout(CancellationToken? cancellationToken) async {
     await Future.delayed(Duration(milliseconds: 200));
     _isLoggedIn = false;
   }
 
   @override
-  void setLoginCredentials(String username, String password) {
+  void setLoginCredentials(Credentials? credentials) {
     // TODO: implement setLoginCredentials
   }
 }
diff --git a/lib/dualis/service/parsing/all_modules_extract.dart b/lib/dualis/service/parsing/all_modules_extract.dart
index 02e76db6..cca84728 100644
--- a/lib/dualis/service/parsing/all_modules_extract.dart
+++ b/lib/dualis/service/parsing/all_modules_extract.dart
@@ -5,7 +5,7 @@ import 'package:html/dom.dart';
 import 'package:html/parser.dart';
 
 class AllModulesExtract {
-  List<DualisModule> extractAllModules(String body) {
+  List<DualisModule> extractAllModules(String? body) {
     try {
       return _extractAllModules(body);
     } catch (e, trace) {
@@ -14,7 +14,7 @@ class AllModulesExtract {
     }
   }
 
-  List<DualisModule> _extractAllModules(String body) {
+  List<DualisModule> _extractAllModules(String? body) {
     var document = parse(body);
 
     var modulesTable = getElementByTagName(document, "tbody");
@@ -49,7 +49,7 @@ class AllModulesExtract {
     var grade = cells[4].innerHtml;
     var state = cells[5].children[0].attributes["alt"];
 
-    ExamState stateEnum;
+    ExamState? stateEnum;
 
     if (state == "Bestanden") {
       stateEnum = ExamState.Passed;
diff --git a/lib/dualis/service/parsing/exams_from_module_details_extract.dart b/lib/dualis/service/parsing/exams_from_module_details_extract.dart
index 769c92be..d90f79e0 100644
--- a/lib/dualis/service/parsing/exams_from_module_details_extract.dart
+++ b/lib/dualis/service/parsing/exams_from_module_details_extract.dart
@@ -4,7 +4,7 @@ import 'package:dhbwstudentapp/dualis/service/parsing/parsing_utils.dart';
 import 'package:html/parser.dart';
 
 class ExamsFromModuleDetailsExtract {
-  List<DualisExam> extractExamsFromModuleDetails(String body) {
+  List<DualisExam> extractExamsFromModuleDetails(String? body) {
     try {
       return _extractExamsFromModuleDetails(body);
     } on ParseException catch (e, trace) {
@@ -13,14 +13,14 @@ class ExamsFromModuleDetailsExtract {
     }
   }
 
-  List<DualisExam> _extractExamsFromModuleDetails(String body) {
+  List<DualisExam> _extractExamsFromModuleDetails(String? body) {
     var document = parse(body);
 
     var tableExams = getElementByTagName(document, "tbody");
     var tableExamsRows = tableExams.getElementsByTagName("tr");
 
-    String currentTry;
-    String currentModule;
+    String? currentTry;
+    String? currentModule;
 
     var exams = <DualisExam>[];
 
diff --git a/lib/dualis/service/parsing/login_redirect_url_extract.dart b/lib/dualis/service/parsing/login_redirect_url_extract.dart
index 847edff5..ea2f2463 100644
--- a/lib/dualis/service/parsing/login_redirect_url_extract.dart
+++ b/lib/dualis/service/parsing/login_redirect_url_extract.dart
@@ -1,12 +1,12 @@
 import 'package:html/parser.dart';
 
 class LoginRedirectUrlExtract {
-  String readRedirectUrl(String body, String redirectUrl) {
+  String? readRedirectUrl(String? body, String redirectUrl) {
     var document = parse(body);
 
     var metaTags = document.getElementsByTagName("meta");
 
-    String redirectContent;
+    String? redirectContent;
 
     for (var metaTag in metaTags) {
       if (!metaTag.attributes.containsKey("http-equiv")) continue;
@@ -23,7 +23,7 @@ class LoginRedirectUrlExtract {
     return getUrlFromHeader(redirectContent, redirectUrl);
   }
 
-  String getUrlFromHeader(String refreshHeader, String endpointUrl) {
+  String? getUrlFromHeader(String? refreshHeader, String endpointUrl) {
     if (refreshHeader == null || !refreshHeader.contains("URL=")) return null;
 
     var refreshHeaderUrlIndex = refreshHeader.indexOf("URL=") + "URL=".length;
diff --git a/lib/dualis/service/parsing/modules_from_course_result_page_extract.dart b/lib/dualis/service/parsing/modules_from_course_result_page_extract.dart
index f903a77f..baa53cb0 100644
--- a/lib/dualis/service/parsing/modules_from_course_result_page_extract.dart
+++ b/lib/dualis/service/parsing/modules_from_course_result_page_extract.dart
@@ -7,8 +7,8 @@ import 'package:html/parser.dart';
 class ModulesFromCourseResultPageExtract {
   final RegExp _extractUrlRegex = RegExp('dl_popUp\\("(.+?)"');
 
-  List<DualisModule> extractModulesFromCourseResultPage(
-    String body,
+  List<DualisModule?> extractModulesFromCourseResultPage(
+    String? body,
     String endpointUrl,
   ) {
     try {
@@ -19,27 +19,27 @@ class ModulesFromCourseResultPageExtract {
     }
   }
 
-  List<DualisModule> _extractModulesFromCourseResultPage(
-      String body, String endpointUrl) {
+  List<DualisModule?> _extractModulesFromCourseResultPage(
+      String? body, String endpointUrl) {
     var document = parse(body);
 
     var tableBodies = getElementByTagName(document, "tbody");
     var rows = tableBodies.getElementsByTagName("tr");
 
-    var modulesOfSemester = <DualisModule>[];
+    var modulesOfSemester = <DualisModule?>[];
 
     for (var row in rows) {
       // Only rows with tds as child are modules
       if (row.children[0].localName != "td") continue;
 
-      DualisModule module = _extractModuleFromRow(row, endpointUrl);
+      DualisModule? module = _extractModuleFromRow(row, endpointUrl);
       modulesOfSemester.add(module);
     }
 
     return modulesOfSemester;
   }
 
-  DualisModule _extractModuleFromRow(
+  DualisModule? _extractModuleFromRow(
     Element row,
     String endpointUrl,
   ) {
@@ -49,13 +49,13 @@ class ModulesFromCourseResultPageExtract {
     var name = row.children[1].innerHtml;
     var grade = row.children[2].innerHtml;
     var credits = row.children[3].innerHtml;
-    var status = row.children[4].innerHtml;
+    String? status = row.children[4].innerHtml;
     var detailsButton = row.children[5];
     var url = _extractDetailsUrlFromButton(detailsButton, endpointUrl);
 
     status = trimAndEscapeString(status);
 
-    ExamState statusEnum;
+    ExamState? statusEnum;
 
     if (status == "bestanden") {
       statusEnum = ExamState.Passed;
@@ -76,7 +76,7 @@ class ModulesFromCourseResultPageExtract {
     return module;
   }
 
-  String _extractDetailsUrlFromButton(
+  String? _extractDetailsUrlFromButton(
     Element detailsButton,
     String endpointUrl,
   ) {
diff --git a/lib/dualis/service/parsing/monthly_schedule_extract.dart b/lib/dualis/service/parsing/monthly_schedule_extract.dart
index d0f8e22a..019a879b 100644
--- a/lib/dualis/service/parsing/monthly_schedule_extract.dart
+++ b/lib/dualis/service/parsing/monthly_schedule_extract.dart
@@ -6,7 +6,7 @@ import 'package:html/parser.dart';
 import 'package:intl/intl.dart';
 
 class MonthlyScheduleExtract {
-  Schedule extractScheduleFromMonthly(String body) {
+  Schedule extractScheduleFromMonthly(String? body) {
     try {
       return _extractScheduleFromMonthly(body);
     } catch (e, trace) {
@@ -15,7 +15,7 @@ class MonthlyScheduleExtract {
     }
   }
 
-  Schedule _extractScheduleFromMonthly(String body) {
+  Schedule _extractScheduleFromMonthly(String? body) {
     var document = parse(body);
 
     var appointments = document.getElementsByClassName("apmntLink");
@@ -29,18 +29,18 @@ class MonthlyScheduleExtract {
     }
 
     allEntries.sort(
-      (ScheduleEntry e1, ScheduleEntry e2) => e1?.start?.compareTo(e2?.start),
+      (ScheduleEntry e1, ScheduleEntry e2) => e1.start.compareTo(e2.start),
     );
 
     return Schedule.fromList(allEntries);
   }
 
   ScheduleEntry _extractEntry(Element appointment) {
-    var date = appointment.parent.parent
-        .querySelector(".tbsubhead a")
+    var date = appointment.parent!.parent!
+        .querySelector(".tbsubhead a")!
         .attributes["title"];
 
-    var information = appointment.attributes["title"];
+    var information = appointment.attributes["title"]!;
     var informationParts = information.split(" / ");
 
     var startAndEnd = informationParts[0].split(" - ");
diff --git a/lib/dualis/service/parsing/parsing_utils.dart b/lib/dualis/service/parsing/parsing_utils.dart
index 4091c44a..e192cca9 100644
--- a/lib/dualis/service/parsing/parsing_utils.dart
+++ b/lib/dualis/service/parsing/parsing_utils.dart
@@ -2,7 +2,7 @@ import 'package:html/dom.dart';
 
 import 'package:universal_html/html.dart' as html;
 
-String trimAndEscapeString(String htmlString) {
+String? trimAndEscapeString(String? htmlString) {
   if (htmlString == null) return null;
 
   var text = html.Element.span()..appendHtml(htmlString);
@@ -45,8 +45,8 @@ Element getElementById(
 }
 
 class ParseException implements Exception {
-  Object innerException;
-  StackTrace trace;
+  Object? innerException;
+  StackTrace? trace;
 
   ParseException.withInner(this.innerException, this.trace);
 
@@ -58,10 +58,10 @@ class ParseException implements Exception {
 
 class ElementNotFoundParseException implements ParseException {
   @override
-  Object innerException;
+  Object? innerException;
 
   @override
-  StackTrace trace;
+  StackTrace? trace;
 
   final String elementDescription;
 
diff --git a/lib/dualis/service/parsing/semesters_from_course_result_page_extract.dart b/lib/dualis/service/parsing/semesters_from_course_result_page_extract.dart
index c6ae1853..2ce424b7 100644
--- a/lib/dualis/service/parsing/semesters_from_course_result_page_extract.dart
+++ b/lib/dualis/service/parsing/semesters_from_course_result_page_extract.dart
@@ -5,7 +5,7 @@ import 'package:html/parser.dart';
 
 class SemestersFromCourseResultPageExtract {
   List<DualisSemester> extractSemestersFromCourseResults(
-    String body,
+    String? body,
     String endpointUrl,
   ) {
     try {
@@ -17,7 +17,7 @@ class SemestersFromCourseResultPageExtract {
   }
 
   List<DualisSemester> _extractSemestersFromCourseResults(
-      String body, String endpointUrl) {
+      String? body, String endpointUrl) {
     var page = parse(body);
 
     var semesterSelector = page.getElementById("semester");
@@ -33,10 +33,10 @@ class SemestersFromCourseResultPageExtract {
       var id = option.attributes["value"];
       var name = option.innerHtml;
 
-      String detailsUrl;
+      String? detailsUrl;
 
       if (url != null) {
-        detailsUrl = url + id;
+        detailsUrl = url + id!;
       }
 
       semesters.add(DualisSemester(
@@ -49,11 +49,11 @@ class SemestersFromCourseResultPageExtract {
     return semesters;
   }
 
-  String _extractSemesterDetailUrlPart(
+  String? _extractSemesterDetailUrlPart(
     Element semesterSelector,
     String endpointUrl,
   ) {
-    var dropDownSemesterSelector = semesterSelector.attributes["onchange"];
+    var dropDownSemesterSelector = semesterSelector.attributes["onchange"]!;
 
     var regExp = RegExp("'([A-z0-9]*)'");
 
diff --git a/lib/dualis/service/parsing/study_grades_from_student_results_page_extract.dart b/lib/dualis/service/parsing/study_grades_from_student_results_page_extract.dart
index 8b6ff079..3af3e401 100644
--- a/lib/dualis/service/parsing/study_grades_from_student_results_page_extract.dart
+++ b/lib/dualis/service/parsing/study_grades_from_student_results_page_extract.dart
@@ -4,7 +4,7 @@ import 'package:html/dom.dart';
 import 'package:html/parser.dart';
 
 class StudyGradesFromStudentResultsPageExtract {
-  StudyGrades extractStudyGradesFromStudentsResultsPage(String body) {
+  StudyGrades extractStudyGradesFromStudentsResultsPage(String? body) {
     try {
       return _extractStudyGradesFromStudentsResultsPage(body);
     } catch (e, trace) {
@@ -13,7 +13,7 @@ class StudyGradesFromStudentResultsPageExtract {
     }
   }
 
-  StudyGrades _extractStudyGradesFromStudentsResultsPage(String body) {
+  StudyGrades _extractStudyGradesFromStudentsResultsPage(String? body) {
     var document = parse(body);
 
     var creditsTable = getElementByTagName(document, "tbody", 0);
@@ -40,12 +40,12 @@ class StudyGradesFromStudentResultsPageExtract {
     var neededCredits = neededCreditsRow.children[0].innerHtml;
 
     // Only take the number after the colon
-    neededCredits = trimAndEscapeString(neededCredits.split(":")[1]);
+    neededCredits = trimAndEscapeString(neededCredits.split(":")[1])!;
 
     var gainedCreditsRow = rows[rows.length - 2];
     var gainedCredits = trimAndEscapeString(
       gainedCreditsRow.children[2].innerHtml,
-    );
+    )!;
 
     neededCredits = neededCredits.replaceAll(",", ".");
     gainedCredits = gainedCredits.replaceAll(",", ".");
@@ -65,12 +65,12 @@ class StudyGradesFromStudentResultsPageExtract {
     var totalGpaRowCells = rows[0].getElementsByTagName("th");
     var totalGpa = trimAndEscapeString(
       totalGpaRowCells[1].innerHtml,
-    );
+    )!;
 
     var mainCoursesGpaRowCells = rows[1].getElementsByTagName("th");
     var mainModulesGpa = trimAndEscapeString(
       mainCoursesGpaRowCells[1].innerHtml,
-    );
+    )!;
 
     _Gpa gpa = _Gpa();
     gpa.totalGpa = double.tryParse(totalGpa.replaceAll(",", "."));
@@ -81,11 +81,11 @@ class StudyGradesFromStudentResultsPageExtract {
 }
 
 class _Credits {
-  double totalCredits;
-  double gainedCredits;
+  double? totalCredits;
+  double? gainedCredits;
 }
 
 class _Gpa {
-  double totalGpa;
-  double mainModulesGpa;
+  double? totalGpa;
+  double? mainModulesGpa;
 }
diff --git a/lib/dualis/service/parsing/urls_from_main_page_extract.dart b/lib/dualis/service/parsing/urls_from_main_page_extract.dart
index 1cea8a03..d8638001 100644
--- a/lib/dualis/service/parsing/urls_from_main_page_extract.dart
+++ b/lib/dualis/service/parsing/urls_from_main_page_extract.dart
@@ -4,7 +4,7 @@ import 'package:html/parser.dart';
 
 class UrlsFromMainPageExtract {
   void parseMainPage(
-    String body,
+    String? body,
     DualisUrls dualsUrls,
     String endpointUrl,
   ) {
@@ -17,7 +17,7 @@ class UrlsFromMainPageExtract {
   }
 
   void _parseMainPage(
-    String body,
+    String? body,
     DualisUrls dualisUrls,
     String endpointUrl,
   ) {
@@ -29,14 +29,14 @@ class UrlsFromMainPageExtract {
     var logoutElement = getElementById(document, "logoutButton");
 
     dualisUrls.courseResultUrl =
-        endpointUrl + courseResultsElement.attributes['href'];
+        endpointUrl + courseResultsElement.attributes['href']!;
 
     dualisUrls.studentResultsUrl =
-        endpointUrl + studentResultsElement.attributes['href'];
+        endpointUrl + studentResultsElement.attributes['href']!;
 
     dualisUrls.monthlyScheduleUrl =
-        endpointUrl + monthlyScheduleElement.attributes["href"];
+        endpointUrl + monthlyScheduleElement.attributes["href"]!;
 
-    dualisUrls.logoutUrl = endpointUrl + logoutElement.attributes['href'];
+    dualisUrls.logoutUrl = endpointUrl + logoutElement.attributes['href']!;
   }
 }
diff --git a/lib/dualis/service/session.dart b/lib/dualis/service/session.dart
index a1563f8d..01d14088 100644
--- a/lib/dualis/service/session.dart
+++ b/lib/dualis/service/session.dart
@@ -5,19 +5,22 @@ import 'package:dhbwstudentapp/schedule/service/schedule_source.dart';
 import 'package:http/http.dart';
 import 'package:http_client_helper/http_client_helper.dart' as http;
 
+// TODO: [Leptopoda] requestCancellationToken and Cancellation token cleanup
+// TODO: [Leptopoda] Pass Uri objects and not strings
+
 ///
 /// Handles cookies and provides a session. Execute your api calls with the
 /// provided get and set methods.
 ///
 class Session {
-  Map<String, String> cookies = {};
+  Map<String, String> _cookies = {};
 
   ///
   /// Execute a GET request and return the result body as string
   ///
-  Future<String> get(
+  Future<String?> get(
     String url, [
-    CancellationToken cancellationToken,
+    CancellationToken? cancellationToken,
   ]) async {
     var response = await rawGet(url, cancellationToken);
 
@@ -32,9 +35,9 @@ class Session {
     }
   }
 
-  Future<Response> rawGet(
+  Future<Response?> rawGet(
     String url, [
-    CancellationToken cancellationToken,
+    CancellationToken? cancellationToken,
   ]) async {
     if (cancellationToken == null) cancellationToken = CancellationToken();
 
@@ -50,13 +53,13 @@ class Session {
       var response = await http.HttpClientHelper.get(
         requestUri,
         cancelToken: requestCancellationToken,
-        headers: cookies,
+        headers: _cookies,
       );
 
       if (response == null && !requestCancellationToken.isCanceled)
         throw ServiceRequestFailed("Http request failed!");
 
-      _updateCookie(response);
+      _updateCookie(response!);
 
       return response;
     } on http.OperationCanceledError catch (_) {
@@ -73,10 +76,10 @@ class Session {
   ///
   /// Execute a POST request and return the result body as string
   ///
-  Future<String> post(
+  Future<String?> post(
     String url,
-    dynamic data, [
-    CancellationToken cancellationToken,
+    Map<String, String> data, [
+    CancellationToken? cancellationToken,
   ]) async {
     var response = await rawPost(url, data, cancellationToken);
 
@@ -91,10 +94,10 @@ class Session {
     }
   }
 
-  Future<Response> rawPost(
+  Future<Response?> rawPost(
     String url,
-    dynamic data, [
-    CancellationToken cancellationToken,
+    Map<String, String> data, [
+    CancellationToken? cancellationToken,
   ]) async {
     if (cancellationToken == null) cancellationToken = CancellationToken();
     var requestCancellationToken = http.CancellationToken();
@@ -107,14 +110,14 @@ class Session {
       var response = await http.HttpClientHelper.post(
         Uri.parse(url),
         body: data,
-        headers: cookies,
+        headers: _cookies,
         cancelToken: requestCancellationToken,
       );
 
       if (response == null && !requestCancellationToken.isCanceled)
         throw ServiceRequestFailed("Http request failed!");
 
-      _updateCookie(response);
+      _updateCookie(response!);
 
       return response;
     } on http.OperationCanceledError catch (_) {
@@ -129,7 +132,7 @@ class Session {
   }
 
   void _updateCookie(Response response) {
-    String rawCookie = response.headers['set-cookie'];
+    String? rawCookie = response.headers['set-cookie'];
     if (rawCookie != null) {
       int index = rawCookie.indexOf(';');
 
@@ -137,7 +140,7 @@ class Session {
 
       cookie = cookie.replaceAll(" ", "");
 
-      cookies['cookie'] = cookie;
+      _cookies['cookie'] = cookie;
     }
   }
 }
diff --git a/lib/dualis/ui/dualis_navigation_entry.dart b/lib/dualis/ui/dualis_navigation_entry.dart
index 95e2097f..3fc5eea4 100644
--- a/lib/dualis/ui/dualis_navigation_entry.dart
+++ b/lib/dualis/ui/dualis_navigation_entry.dart
@@ -1,6 +1,5 @@
 import 'package:dhbwstudentapp/common/i18n/localizations.dart';
 import 'package:dhbwstudentapp/common/ui/custom_icons_icons.dart';
-import 'package:dhbwstudentapp/common/ui/viewmodels/base_view_model.dart';
 import 'package:dhbwstudentapp/dualis/ui/dualis_page.dart';
 import 'package:dhbwstudentapp/dualis/ui/viewmodels/study_grades_view_model.dart';
 import 'package:dhbwstudentapp/dualis/ui/widgets/dualis_help_dialog.dart';
@@ -9,13 +8,9 @@ import 'package:flutter/material.dart';
 import 'package:kiwi/kiwi.dart';
 import 'package:property_change_notifier/property_change_notifier.dart';
 
-class DualisNavigationEntry extends NavigationEntry {
-  StudyGradesViewModel _viewModel;
-
+class DualisNavigationEntry extends NavigationEntry<StudyGradesViewModel> {
   @override
-  Widget icon(BuildContext context) {
-    return Icon(Icons.data_usage);
-  }
+  Icon icon = Icon(Icons.data_usage);
 
   @override
   String title(BuildContext context) {
@@ -23,15 +18,11 @@ class DualisNavigationEntry extends NavigationEntry {
   }
 
   @override
-  BaseViewModel initViewModel() {
-    if (_viewModel == null) {
-      _viewModel = StudyGradesViewModel(
-        KiwiContainer().resolve(),
-        KiwiContainer().resolve(),
-      );
-    }
-
-    return _viewModel;
+  StudyGradesViewModel initViewModel() {
+    return StudyGradesViewModel(
+      KiwiContainer().resolve(),
+      KiwiContainer().resolve(),
+    );
   }
 
   @override
@@ -41,27 +32,27 @@ class DualisNavigationEntry extends NavigationEntry {
 
   @override
   List<Widget> appBarActions(BuildContext context) {
-    initViewModel();
     return [
       PropertyChangeProvider<StudyGradesViewModel, Object>(
-        value: _viewModel,
+        value: model,
         child: PropertyChangeConsumer(
-          builder: (BuildContext _, StudyGradesViewModel __, Set<Object> ___) =>
-              _viewModel.loginState != LoginState.LoggedIn
-                  ? IconButton(
-                      icon: Icon(Icons.help_outline),
-                      onPressed: () async {
-                        await DualisHelpDialog().show(context);
-                      },
-                      tooltip: L.of(context).helpButtonTooltip,
-                    )
-                  : IconButton(
-                      icon: const Icon(CustomIcons.logout),
-                      onPressed: () async {
-                        await _viewModel.logout();
-                      },
-                      tooltip: L.of(context).logoutButtonTooltip,
-                    ),
+          builder:
+              (BuildContext _, StudyGradesViewModel? __, Set<Object>? ___) =>
+                  model.loginState != LoginState.LoggedIn
+                      ? IconButton(
+                          icon: Icon(Icons.help_outline),
+                          onPressed: () async {
+                            await DualisHelpDialog().show(context);
+                          },
+                          tooltip: L.of(context).helpButtonTooltip,
+                        )
+                      : IconButton(
+                          icon: const Icon(CustomIcons.logout),
+                          onPressed: () async {
+                            await model.logout();
+                          },
+                          tooltip: L.of(context).logoutButtonTooltip,
+                        ),
         ),
       ),
     ];
diff --git a/lib/dualis/ui/dualis_page.dart b/lib/dualis/ui/dualis_page.dart
index 5f866d55..cd9e017c 100644
--- a/lib/dualis/ui/dualis_page.dart
+++ b/lib/dualis/ui/dualis_page.dart
@@ -1,5 +1,4 @@
 import 'package:dhbwstudentapp/common/i18n/localizations.dart';
-import 'package:dhbwstudentapp/common/ui/viewmodels/base_view_model.dart';
 import 'package:dhbwstudentapp/dualis/ui/exam_results_page/exam_results_page.dart';
 import 'package:dhbwstudentapp/dualis/ui/login/dualis_login_page.dart';
 import 'package:dhbwstudentapp/dualis/ui/study_overview/study_overview_page.dart';
@@ -12,7 +11,7 @@ import 'package:provider/provider.dart';
 class DualisPage extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
-    StudyGradesViewModel viewModel = Provider.of<BaseViewModel>(context);
+    StudyGradesViewModel viewModel = Provider.of<StudyGradesViewModel>(context);
 
     Widget widget;
 
diff --git a/lib/dualis/ui/exam_results_page/exam_results_page.dart b/lib/dualis/ui/exam_results_page/exam_results_page.dart
index 315dc141..25f09e4f 100644
--- a/lib/dualis/ui/exam_results_page/exam_results_page.dart
+++ b/lib/dualis/ui/exam_results_page/exam_results_page.dart
@@ -38,14 +38,14 @@ class ExamResultsPage extends StatelessWidget {
                       ],
                       builder: (
                         BuildContext context,
-                        StudyGradesViewModel model,
-                        Set properties,
+                        StudyGradesViewModel? model,
+                        Set? properties,
                       ) =>
                           DropdownButton(
-                        onChanged: (value) => model.loadSemester(value),
-                        value: model.currentSemesterName,
-                        items: (model.allSemesterNames ?? [])
-                            .map(
+                        onChanged: model?.loadSemester,
+                        value: model?.currentSemesterName,
+                        items: model?.allSemesterNames
+                            ?.map(
                               (n) => DropdownMenuItem(
                                 child: Text(n),
                                 value: n,
@@ -62,10 +62,10 @@ class ExamResultsPage extends StatelessWidget {
               properties: const ["currentSemester"],
               builder: (
                 BuildContext context,
-                StudyGradesViewModel model,
-                Set properties,
+                StudyGradesViewModel? model,
+                Set? properties,
               ) =>
-                  model.currentSemester != null
+                  model!.currentSemester != null
                       ? buildModulesColumn(context, model)
                       : const Padding(
                           padding: EdgeInsets.fromLTRB(0, 16, 0, 0),
@@ -81,7 +81,7 @@ class ExamResultsPage extends StatelessWidget {
   Widget buildModulesColumn(
       BuildContext context, StudyGradesViewModel viewModel) {
     return AnimatedSwitcher(
-      layoutBuilder: (Widget currentChild, List<Widget> previousChildren) {
+      layoutBuilder: (Widget? currentChild, List<Widget> previousChildren) {
         List<Widget> children = previousChildren;
         if (currentChild != null)
           children = children.toList()..add(currentChild);
@@ -103,7 +103,7 @@ class ExamResultsPage extends StatelessWidget {
     var dataTables = <Widget>[];
 
     var isFirstModule = true;
-    for (var module in viewModel.currentSemester.modules) {
+    for (var module in viewModel.currentSemester!.modules) {
       dataTables.add(DataTable(
         horizontalMargin: 24,
         columnSpacing: 0,
@@ -157,14 +157,12 @@ class ExamResultsPage extends StatelessWidget {
       case ExamGradeState.NotGraded:
         return Text("");
       case ExamGradeState.Graded:
-        return Text(grade.gradeValue);
+        return Text(grade.gradeValue!);
       case ExamGradeState.Passed:
         return Text(L.of(context).examPassed);
       case ExamGradeState.Failed:
         return Text(L.of(context).examNotPassed);
     }
-
-    return Text("");
   }
 
   List<DataColumn> buildModuleColumns(BuildContext context, var module,
diff --git a/lib/dualis/ui/login/dualis_login_page.dart b/lib/dualis/ui/login/dualis_login_page.dart
index 204dd8a2..3fd8ff86 100644
--- a/lib/dualis/ui/login/dualis_login_page.dart
+++ b/lib/dualis/ui/login/dualis_login_page.dart
@@ -1,5 +1,4 @@
 import 'package:dhbwstudentapp/common/i18n/localizations.dart';
-import 'package:dhbwstudentapp/common/ui/viewmodels/base_view_model.dart';
 import 'package:dhbwstudentapp/dualis/ui/viewmodels/study_grades_view_model.dart';
 import 'package:dhbwstudentapp/dualis/ui/widgets/login_form_widget.dart';
 import 'package:flutter/material.dart';
@@ -8,7 +7,7 @@ import 'package:provider/provider.dart';
 class DualisLoginPage extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
-    StudyGradesViewModel viewModel = Provider.of<BaseViewModel>(context);
+    StudyGradesViewModel viewModel = Provider.of<StudyGradesViewModel>(context);
     return buildLoginPage(context, viewModel);
   }
 
diff --git a/lib/dualis/ui/study_overview/study_overview_page.dart b/lib/dualis/ui/study_overview/study_overview_page.dart
index 1cd0f267..fd6a8a4c 100644
--- a/lib/dualis/ui/study_overview/study_overview_page.dart
+++ b/lib/dualis/ui/study_overview/study_overview_page.dart
@@ -39,10 +39,10 @@ class StudyOverviewPage extends StatelessWidget {
             properties: const ["studyGrades"],
             builder: (
               BuildContext context,
-              StudyGradesViewModel model,
-              Set properties,
+              StudyGradesViewModel? model,
+              Set? properties,
             ) =>
-                model.studyGrades != null
+                model!.studyGrades != null
                     ? Column(
                         mainAxisAlignment: MainAxisAlignment.start,
                         crossAxisAlignment: CrossAxisAlignment.stretch,
@@ -54,7 +54,7 @@ class StudyOverviewPage extends StatelessWidget {
                               crossAxisAlignment: CrossAxisAlignment.baseline,
                               children: <Widget>[
                                 Text(
-                                  model.studyGrades.gpaTotal.toString(),
+                                  model.studyGrades!.gpaTotal.toString(),
                                   style: Theme.of(context).textTheme.headline3,
                                 ),
                                 Padding(
@@ -72,7 +72,7 @@ class StudyOverviewPage extends StatelessWidget {
                             crossAxisAlignment: CrossAxisAlignment.baseline,
                             children: <Widget>[
                               Text(
-                                model.studyGrades.gpaMainModules.toString(),
+                                model.studyGrades!.gpaMainModules.toString(),
                                 style: Theme.of(context).textTheme.headline3,
                               ),
                               Padding(
@@ -90,7 +90,7 @@ class StudyOverviewPage extends StatelessWidget {
                               crossAxisAlignment: CrossAxisAlignment.baseline,
                               children: <Widget>[
                                 Text(
-                                  "${model.studyGrades.creditsGained} / ${model.studyGrades.creditsTotal}",
+                                  "${model.studyGrades!.creditsGained} / ${model.studyGrades!.creditsTotal}",
                                   style: Theme.of(context).textTheme.headline3,
                                 ),
                                 Padding(
@@ -129,10 +129,10 @@ class StudyOverviewPage extends StatelessWidget {
             properties: const ["allModules"],
             builder: (
               BuildContext context,
-              StudyGradesViewModel model,
-              Set properties,
+              StudyGradesViewModel? model,
+              Set? properties,
             ) =>
-                model.allModules != null
+                model!.allModules != null
                     ? buildModulesDataTable(context, model)
                     : buildProgressIndicator(),
           ),
@@ -154,13 +154,13 @@ class StudyOverviewPage extends StatelessWidget {
   ) {
     var dataRows = <DataRow>[];
 
-    for (var module in model.allModules) {
+    for (var module in model.allModules!) {
       dataRows.add(
         DataRow(
           cells: <DataCell>[
-            DataCell(Text(module.name)),
-            DataCell(Text(module.credits)),
-            DataCell(Text(module.grade)),
+            DataCell(Text(module.name!)),
+            DataCell(Text(module.credits!)),
+            DataCell(Text(module.grade!)),
             DataCell(GradeStateIcon(state: module.state)),
           ],
         ),
diff --git a/lib/dualis/ui/viewmodels/study_grades_view_model.dart b/lib/dualis/ui/viewmodels/study_grades_view_model.dart
index 2b7305b1..32251a4f 100644
--- a/lib/dualis/ui/viewmodels/study_grades_view_model.dart
+++ b/lib/dualis/ui/viewmodels/study_grades_view_model.dart
@@ -25,26 +25,26 @@ class StudyGradesViewModel extends BaseViewModel {
   LoginState _loginState = LoginState.LoggedOut;
   LoginState get loginState => _loginState;
 
-  StudyGrades _studyGrades;
-  StudyGrades get studyGrades => _studyGrades;
+  StudyGrades? _studyGrades;
+  StudyGrades? get studyGrades => _studyGrades;
   final CancelableMutex _studyGradesCancellationToken = CancelableMutex();
 
-  List<Module> _allModules;
-  List<Module> get allModules => _allModules;
+  List<Module>? _allModules;
+  List<Module>? get allModules => _allModules;
   final CancelableMutex _allModulesCancellationToken = CancelableMutex();
 
-  List<String> _semesterNames;
-  List<String> get allSemesterNames => _semesterNames;
+  List<String>? _semesterNames;
+  List<String>? get allSemesterNames => _semesterNames;
   final CancelableMutex _semesterNamesCancellationToken = CancelableMutex();
 
-  Semester _currentSemester;
-  Semester get currentSemester => _currentSemester;
+  Semester? _currentSemester;
+  Semester? get currentSemester => _currentSemester;
   final CancelableMutex _currentSemesterCancellationToken = CancelableMutex();
 
-  String _currentSemesterName;
-  String get currentSemesterName => _currentSemesterName;
+  String? _currentSemesterName;
+  String? get currentSemesterName => _currentSemesterName;
 
-  String _currentLoadingSemesterName;
+  String? _currentLoadingSemesterName;
 
   StudyGradesViewModel(this._preferencesProvider, this._dualisService);
 
@@ -54,10 +54,7 @@ class StudyGradesViewModel extends BaseViewModel {
     bool success;
 
     try {
-      var result = await _dualisService.login(
-        credentials.username,
-        credentials.password,
-      );
+      var result = await _dualisService.login(credentials);
 
       success = result == LoginResult.LoggedIn;
     } on OperationCancelledException catch (_) {
@@ -95,7 +92,7 @@ class StudyGradesViewModel extends BaseViewModel {
     await _preferencesProvider.clearDualisCredentials();
   }
 
-  Future<Credentials> loadCredentials() async {
+  Future<Credentials?> loadCredentials() async {
     return await _preferencesProvider.loadDualisCredentials();
   }
 
@@ -109,7 +106,7 @@ class StudyGradesViewModel extends BaseViewModel {
   }
 
   Future<void> loadStudyGrades() async {
-    if (_studyGrades != null) return Future.value();
+    if (_studyGrades != null) return;
 
     print("Loading study grades...");
 
@@ -130,7 +127,7 @@ class StudyGradesViewModel extends BaseViewModel {
   }
 
   Future<void> loadAllModules() async {
-    if (_allModules != null) return Future.value();
+    if (_allModules != null) return;
 
     print("Loading all modules...");
 
@@ -151,7 +148,7 @@ class StudyGradesViewModel extends BaseViewModel {
     notifyListeners("allModules");
   }
 
-  Future<void> loadSemester(String semesterName) async {
+  Future<void> loadSemester(String? semesterName) async {
     if (_currentSemester != null && _currentSemesterName == semesterName)
       return Future.value();
 
@@ -186,7 +183,7 @@ class StudyGradesViewModel extends BaseViewModel {
   }
 
   Future<void> loadSemesterNames() async {
-    if (_semesterNames != null) return Future.value();
+    if (_semesterNames != null) return;
 
     print("Loading semester names...");
 
@@ -211,14 +208,14 @@ class StudyGradesViewModel extends BaseViewModel {
 
   Future _loadInitialSemester() async {
     if (_semesterNames == null) return;
-    if (_semesterNames.isEmpty) return;
+    if (_semesterNames!.isEmpty) return;
 
     var lastViewedSemester = await _preferencesProvider.getLastViewedSemester();
 
-    if (_semesterNames.contains(lastViewedSemester)) {
+    if (_semesterNames!.contains(lastViewedSemester)) {
       loadSemester(lastViewedSemester);
     } else {
-      loadSemester(_semesterNames.first);
+      loadSemester(_semesterNames!.first);
     }
   }
 
diff --git a/lib/dualis/ui/widgets/grade_state_icon.dart b/lib/dualis/ui/widgets/grade_state_icon.dart
index 8c3e1236..5984b6e4 100644
--- a/lib/dualis/ui/widgets/grade_state_icon.dart
+++ b/lib/dualis/ui/widgets/grade_state_icon.dart
@@ -2,11 +2,11 @@ import 'package:dhbwstudentapp/dualis/model/exam.dart';
 import 'package:flutter/material.dart';
 
 class GradeStateIcon extends StatelessWidget {
-  final ExamState state;
+  final ExamState? state;
 
   const GradeStateIcon({
-    Key key,
-    this.state,
+    Key? key,
+    required this.state,
   }) : super(key: key);
 
   @override
@@ -17,9 +17,8 @@ class GradeStateIcon extends StatelessWidget {
       case ExamState.Failed:
         return const Icon(Icons.close, color: Colors.red);
       case ExamState.Pending:
+      default:
         return Container();
     }
-
-    return Container();
   }
 }
diff --git a/lib/dualis/ui/widgets/login_form_widget.dart b/lib/dualis/ui/widgets/login_form_widget.dart
index 2e71236e..7d061a64 100644
--- a/lib/dualis/ui/widgets/login_form_widget.dart
+++ b/lib/dualis/ui/widgets/login_form_widget.dart
@@ -3,7 +3,7 @@ import 'package:dhbwstudentapp/dualis/model/credentials.dart';
 import 'package:flutter/material.dart';
 
 typedef OnLogin = Future<bool> Function(Credentials credentials);
-typedef OnLoadCredentials = Future<Credentials> Function();
+typedef OnLoadCredentials = Future<Credentials?> Function();
 typedef OnSaveCredentials = Future<void> Function(Credentials credentials);
 typedef OnClearCredentials = Future<void> Function();
 typedef GetDoSaveCredentials = Future<bool> Function();
@@ -19,77 +19,52 @@ class LoginForm extends StatefulWidget {
   final String loginFailedText;
 
   const LoginForm({
-    Key key,
-    this.onLogin,
-    this.title,
-    this.loginFailedText,
-    this.onLoadCredentials,
-    this.onSaveCredentials,
-    this.onClearCredentials,
-    this.getDoSaveCredentials,
+    Key? key,
+    required this.onLogin,
+    required this.title,
+    required this.loginFailedText,
+    required this.onLoadCredentials,
+    required this.onSaveCredentials,
+    required this.onClearCredentials,
+    required this.getDoSaveCredentials,
   }) : super(key: key);
 
   @override
-  _LoginFormState createState() => _LoginFormState(
-        onLogin,
-        title,
-        loginFailedText,
-        onLoadCredentials,
-        onSaveCredentials,
-        onClearCredentials,
-        getDoSaveCredentials,
-      );
+  _LoginFormState createState() => _LoginFormState();
 }
 
 class _LoginFormState extends State<LoginForm> {
-  final OnLogin _onLogin;
-  final OnLoadCredentials _onLoadCredentials;
-  final OnSaveCredentials _onSaveCredentials;
-  final OnClearCredentials _onClearCredentials;
-  final GetDoSaveCredentials _getDoSaveCredentials;
-  final Widget _title;
-
-  final String _loginFailedText;
-
   bool _storeCredentials = false;
   bool _loginFailed = false;
   bool _isLoading = false;
 
-  final TextEditingController _usernameEditingController =
-      TextEditingController();
-  final TextEditingController _passwordEditingController =
-      TextEditingController();
-
-  _LoginFormState(
-    this._onLogin,
-    this._title,
-    this._loginFailedText,
-    this._onLoadCredentials,
-    this._onSaveCredentials,
-    this._onClearCredentials,
-    this._getDoSaveCredentials,
-  );
+  final CredentialsEditingController _controller =
+      CredentialsEditingController();
 
   @override
   void initState() {
     super.initState();
 
-    if (_onLoadCredentials != null && _getDoSaveCredentials != null) {
-      _getDoSaveCredentials().then((value) {
-        setState(() {
-          _storeCredentials = value;
-        });
-
-        _onLoadCredentials().then((value) {
-          _usernameEditingController.text = value.username;
-          _passwordEditingController.text = value.password;
+    widget.getDoSaveCredentials().then((value) {
+      setState(() {
+        _storeCredentials = value;
+      });
 
+      widget.onLoadCredentials().then((credentials) {
+        if (credentials != null) {
+          _controller.credentials = credentials;
           if (mounted) {
             setState(() {});
           }
-        });
+        }
       });
-    }
+    });
+  }
+
+  @override
+  void dispose() {
+    _controller.dispose();
+    super.dispose();
   }
 
   @override
@@ -98,14 +73,12 @@ class _LoginFormState extends State<LoginForm> {
       crossAxisAlignment: CrossAxisAlignment.start,
       mainAxisSize: MainAxisSize.min,
       children: <Widget>[
-        _title != null
-            ? Padding(
-                padding: const EdgeInsets.fromLTRB(0, 0, 0, 24),
-                child: _title,
-              )
-            : Container(),
+        Padding(
+          padding: const EdgeInsets.fromLTRB(0, 0, 0, 24),
+          child: widget.title,
+        ),
         TextField(
-          controller: _usernameEditingController,
+          controller: _controller.username,
           decoration: InputDecoration(
             enabled: !_isLoading,
             hintText: L.of(context).loginUsername,
@@ -113,13 +86,13 @@ class _LoginFormState extends State<LoginForm> {
           ),
         ),
         TextField(
-          controller: _passwordEditingController,
+          controller: _controller.password,
           obscureText: true,
           decoration: InputDecoration(
             enabled: !_isLoading,
             hintText: L.of(context).loginPassword,
             icon: Icon(Icons.lock_outline),
-            errorText: _loginFailed ? _loginFailedText : null,
+            errorText: _loginFailed ? widget.loginFailedText : null,
           ),
         ),
         Padding(
@@ -130,7 +103,8 @@ class _LoginFormState extends State<LoginForm> {
             title: Text(
               L.of(context).dualisStoreCredentials,
             ),
-            onChanged: (bool value) {
+            onChanged: (bool? value) {
+              if (value == null) return;
               setState(() {
                 _storeCredentials = value;
               });
@@ -165,19 +139,16 @@ class _LoginFormState extends State<LoginForm> {
       _isLoading = true;
     });
 
-    if (!_storeCredentials && _onClearCredentials != null) {
-      await _onClearCredentials();
+    if (!_storeCredentials) {
+      await widget.onClearCredentials();
     }
 
-    var credentials = Credentials(
-      _usernameEditingController.text,
-      _passwordEditingController.text,
-    );
+    var credentials = _controller.credentials;
 
-    bool loginSuccess = await _onLogin(credentials);
+    bool loginSuccess = await widget.onLogin(credentials);
 
-    if (loginSuccess && _storeCredentials && _onSaveCredentials != null) {
-      await _onSaveCredentials(credentials);
+    if (loginSuccess && _storeCredentials) {
+      await widget.onSaveCredentials(credentials);
     }
 
     setState(() {
diff --git a/lib/information/ui/useful_information_navigation_entry.dart b/lib/information/ui/useful_information_navigation_entry.dart
index 14271bf5..08713249 100644
--- a/lib/information/ui/useful_information_navigation_entry.dart
+++ b/lib/information/ui/useful_information_navigation_entry.dart
@@ -1,18 +1,17 @@
 import 'package:dhbwstudentapp/common/i18n/localizations.dart';
+import 'package:dhbwstudentapp/common/ui/viewmodels/base_view_model.dart';
 import 'package:dhbwstudentapp/information/ui/usefulinformation/useful_information_page.dart';
 import 'package:dhbwstudentapp/ui/navigation/navigation_entry.dart';
 import 'package:flutter/material.dart';
 
-class UsefulInformationNavigationEntry extends NavigationEntry {
+class UsefulInformationNavigationEntry extends NavigationEntry<BaseViewModel> {
   @override
   Widget build(BuildContext context) {
     return UsefulInformationPage();
   }
 
   @override
-  Widget icon(BuildContext context) {
-    return Icon(Icons.info_outline);
-  }
+  Icon icon = Icon(Icons.info_outline);
 
   @override
   String title(BuildContext context) {
@@ -21,4 +20,10 @@ class UsefulInformationNavigationEntry extends NavigationEntry {
 
   @override
   String get route => "usefulInformation";
+
+  @override
+  BaseViewModel initViewModel() => BaseViewModel();
+
+  @override
+  List<Widget> appBarActions(BuildContext context) => [];
 }
diff --git a/lib/information/ui/usefulinformation/useful_information_page.dart b/lib/information/ui/usefulinformation/useful_information_page.dart
index 903db7af..d480dd32 100644
--- a/lib/information/ui/usefulinformation/useful_information_page.dart
+++ b/lib/information/ui/usefulinformation/useful_information_page.dart
@@ -26,7 +26,8 @@ class UsefulInformationPage extends StatelessWidget {
             leading: Icon(Icons.email),
             title: Text(L.of(context).informationPageRoundcube),
             onTap: () {
-              openLink("https://lehre-webmail.dhbw-stuttgart.de/roundcubemail/");
+              openLink(
+                  "https://lehre-webmail.dhbw-stuttgart.de/roundcubemail/");
             },
           ),
           ListTile(
diff --git a/lib/native/widget/android_widget_helper.dart b/lib/native/widget/android_widget_helper.dart
index 464d69de..a193aac1 100644
--- a/lib/native/widget/android_widget_helper.dart
+++ b/lib/native/widget/android_widget_helper.dart
@@ -30,7 +30,7 @@ class AndroidWidgetHelper implements WidgetHelper {
   }
 
   @override
-  Future<bool> areWidgetsSupported() async {
+  Future<bool?> areWidgetsSupported() async {
     try {
       return await platform.invokeMethod('areWidgetsSupported');
     } on Exception catch (_) {
diff --git a/lib/native/widget/ios_widget_helper.dart b/lib/native/widget/ios_widget_helper.dart
index 7e4a2b2e..0aa61fb8 100644
--- a/lib/native/widget/ios_widget_helper.dart
+++ b/lib/native/widget/ios_widget_helper.dart
@@ -32,7 +32,7 @@ class IOSWidgetHelper implements WidgetHelper {
   }
 
   @override
-  Future<bool> areWidgetsSupported() async {
+  Future<bool?> areWidgetsSupported() async {
     try {
       return await platform.invokeMethod('areWidgetsSupported');
     } on PlatformException catch (_) {
diff --git a/lib/native/widget/widget_helper.dart b/lib/native/widget/widget_helper.dart
index 8b8e98ad..e0f03028 100644
--- a/lib/native/widget/widget_helper.dart
+++ b/lib/native/widget/widget_helper.dart
@@ -8,7 +8,7 @@ import 'package:dhbwstudentapp/native/widget/ios_widget_helper.dart';
 /// methods to enable/disable or update the widget
 ///
 class WidgetHelper {
-  static WidgetHelper _instance;
+  static WidgetHelper? _instance;
 
   WidgetHelper() {
     if (_instance != null) return;
@@ -27,14 +27,14 @@ class WidgetHelper {
   /// scheduled and will happen soon.
   ///
   Future<void> requestWidgetRefresh() {
-    return _instance.requestWidgetRefresh();
+    return _instance!.requestWidgetRefresh();
   }
 
   ///
   /// Checks if widgets are supported by the device
   ///
-  Future<bool> areWidgetsSupported() {
-    return _instance.areWidgetsSupported();
+  Future<bool?> areWidgetsSupported() {
+    return _instance!.areWidgetsSupported();
   }
 
   ///
@@ -42,7 +42,7 @@ class WidgetHelper {
   /// its full functionality.
   ///
   Future<void> enableWidget() {
-    return _instance.enableWidget();
+    return _instance!.enableWidget();
   }
 
   ///
@@ -50,7 +50,7 @@ class WidgetHelper {
   /// only provide placeholder content or limited functionality.
   ///
   Future<void> disableWidget() {
-    return _instance.disableWidget();
+    return _instance!.disableWidget();
   }
 }
 
diff --git a/lib/schedule/background/background_schedule_update.dart b/lib/schedule/background/background_schedule_update.dart
index 7c63d1a6..c8106b8b 100644
--- a/lib/schedule/background/background_schedule_update.dart
+++ b/lib/schedule/background/background_schedule_update.dart
@@ -22,8 +22,8 @@ class BackgroundScheduleUpdate extends TaskCallback {
       return;
     }
 
-    var today = toDayOfWeek(toStartOfDay(DateTime.now()), DateTime.monday);
-    var end = addDays(today, 7 * 3);
+    var today = toDayOfWeek(toStartOfDay(DateTime.now()), DateTime.monday)!;
+    var end = addDays(today, 7 * 3)!;
 
     var cancellationToken = CancellationToken();
 
diff --git a/lib/schedule/background/calendar_synchronizer.dart b/lib/schedule/background/calendar_synchronizer.dart
index 158f89d2..f67ef0b4 100644
--- a/lib/schedule/background/calendar_synchronizer.dart
+++ b/lib/schedule/background/calendar_synchronizer.dart
@@ -35,7 +35,7 @@ class CalendarSynchronizer {
           listDateEntries;
 
       if (await preferencesProvider.isCalendarSyncEnabled()) {
-        Calendar selectedCalendar =
+        Calendar? selectedCalendar =
             await preferencesProvider.getSelectedCalendar();
         if (selectedCalendar == null) return;
         CalendarAccess().addOrUpdateDates(listDateEntries, selectedCalendar);
@@ -47,7 +47,6 @@ class CalendarSynchronizer {
     Future.delayed(Duration(seconds: 10), () {
       if (!scheduleSourceProvider.didSetupCorrectly()) return;
       scheduleProvider.getUpdatedSchedule(
-        
         DateTime.now(),
         DateTime.now().add(Duration(days: 30)),
         CancellationToken(),
diff --git a/lib/schedule/business/schedule_diff_calculator.dart b/lib/schedule/business/schedule_diff_calculator.dart
index 077bdc17..c8be9661 100644
--- a/lib/schedule/business/schedule_diff_calculator.dart
+++ b/lib/schedule/business/schedule_diff_calculator.dart
@@ -39,17 +39,17 @@ class ScheduleDiffCalculator {
     return entry1.start.isAtSameMomentAs(entry2.start) &&
         entry1.end.isAtSameMomentAs(entry2.end) &&
         entry1.type == entry2.type &&
-        (entry1.room ?? "") == (entry2.room ?? "") &&
-        (entry1.details ?? "") == (entry2.details ?? "") &&
-        (entry1.title ?? "") == (entry2.title ?? "") &&
-        (entry1.professor ?? "") == (entry2.professor ?? "");
+        entry1.room == entry2.room &&
+        entry1.details == entry2.details &&
+        entry1.title == entry2.title &&
+        entry1.professor == entry2.professor;
   }
 
   ScheduleDiff _tryConnectNewAndOldEntries(
     List<ScheduleEntry> addedEntries,
     List<ScheduleEntry> removedEntries,
   ) {
-    var allDistinctTitles = <String>[];
+    var allDistinctTitles = <String?>[];
 
     allDistinctTitles.addAll(addedEntries.map((ScheduleEntry e) => e.title));
     allDistinctTitles.addAll(removedEntries.map((ScheduleEntry e) => e.title));
@@ -166,7 +166,10 @@ class ScheduleDiff {
   final List<ScheduleEntry> removedEntries;
   final List<UpdatedEntry> updatedEntries;
 
-  ScheduleDiff({this.addedEntries, this.removedEntries, this.updatedEntries});
+  ScheduleDiff(
+      {required this.addedEntries,
+      required this.removedEntries,
+      required this.updatedEntries});
 
   bool didSomethingChange() {
     return addedEntries.isNotEmpty ||
diff --git a/lib/schedule/business/schedule_provider.dart b/lib/schedule/business/schedule_provider.dart
index 69d7b426..0dee19a6 100644
--- a/lib/schedule/business/schedule_provider.dart
+++ b/lib/schedule/business/schedule_provider.dart
@@ -33,7 +33,7 @@ class ScheduleProvider {
   final List<ScheduleUpdatedCallback> _scheduleUpdatedCallbacks =
       <ScheduleUpdatedCallback>[];
 
-  ScheduleFilter _scheduleFilter;
+  late ScheduleFilter _scheduleFilter;
 
   final List<ScheduleEntryChangedCallback> _scheduleEntryChangedCallbacks =
       <ScheduleEntryChangedCallback>[];
@@ -73,7 +73,7 @@ class ScheduleProvider {
       var updatedSchedule = await _scheduleSource.currentScheduleSource
           .querySchedule(start, end, cancellationToken);
 
-      var schedule = updatedSchedule.schedule;
+      var schedule = updatedSchedule?.schedule;
 
       if (schedule == null) {
         print("No schedule returned!");
@@ -97,10 +97,10 @@ class ScheduleProvider {
       }
 
       for (var c in _scheduleUpdatedCallbacks) {
-        await c(schedule, start, end);
+        await c(schedule!, start, end);
       }
 
-      updatedSchedule = ScheduleQueryResult(schedule, updatedSchedule.errors);
+      updatedSchedule = ScheduleQueryResult(schedule!, updatedSchedule!.errors);
 
       return updatedSchedule;
     } on ScheduleQueryFailedException catch (e, trace) {
@@ -162,7 +162,7 @@ class ScheduleProvider {
 
     for (var addedEntry in diff.addedEntries) {
       if (queryInformation.any((i) =>
-          addedEntry.end.isAfter(i.start) &&
+          addedEntry.end.isAfter(i!.start) &&
           addedEntry.start.isBefore(i.end))) {
         cleanedAddedEntries.add(addedEntry);
       }
diff --git a/lib/schedule/business/schedule_source_provider.dart b/lib/schedule/business/schedule_source_provider.dart
index 06e244b9..4b0a1ff4 100644
--- a/lib/schedule/business/schedule_source_provider.dart
+++ b/lib/schedule/business/schedule_source_provider.dart
@@ -55,7 +55,7 @@ class ScheduleSourceProvider {
     };
 
     if (initializer.containsKey(scheduleSourceType)) {
-      scheduleSource = await initializer[scheduleSourceType]();
+      scheduleSource = await initializer[scheduleSourceType]!();
     }
 
     _currentScheduleSource = scheduleSource;
@@ -72,11 +72,7 @@ class ScheduleSourceProvider {
   Future<ScheduleSourceType> _getScheduleSourceType() async {
     var type = await _preferencesProvider.getScheduleSourceType();
 
-    var scheduleSourceType = type != null
-        ? ScheduleSourceType.values[type]
-        : ScheduleSourceType.None;
-
-    return scheduleSourceType;
+    return ScheduleSourceType.values[type];
   }
 
   Future<ScheduleSource> _dualisScheduleSource() async {
@@ -84,7 +80,7 @@ class ScheduleSourceProvider {
 
     var credentials = await _preferencesProvider.loadDualisCredentials();
 
-    if (credentials.allFieldsFilled()) {
+    if (credentials != null) {
       dualis.setLoginCredentials(credentials);
       return ErrorReportScheduleSourceDecorator(dualis);
     } else {
@@ -130,7 +126,9 @@ class ScheduleSourceProvider {
     return InvalidScheduleSource();
   }
 
-  Future<void> setupForRapla(String url) async {
+  Future<void> setupForRapla(String? url) async {
+    if (url == null) return;
+
     await _preferencesProvider.setRaplaUrl(url);
     await _preferencesProvider
         .setScheduleSourceType(ScheduleSourceType.Rapla.index);
@@ -157,7 +155,9 @@ class ScheduleSourceProvider {
     );
   }
 
-  Future<void> setupForIcal(String url) async {
+  Future<void> setupForIcal(String? url) async {
+    if (url == null) return;
+
     await _preferencesProvider.setIcalUrl(url);
     await _preferencesProvider
         .setScheduleSourceType(ScheduleSourceType.Ical.index);
@@ -171,8 +171,8 @@ class ScheduleSourceProvider {
     );
   }
 
-  Future<void> setupForMannheim(Course selectedCourse) async {
-    if(selectedCourse == null) return;
+  Future<void> setupForMannheim(Course? selectedCourse) async {
+    if (selectedCourse == null) return;
     await _preferencesProvider.setMannheimScheduleId(selectedCourse.scheduleId);
     await _preferencesProvider.setIcalUrl(selectedCourse.icalUrl);
     await _preferencesProvider
@@ -188,8 +188,7 @@ class ScheduleSourceProvider {
   }
 
   bool didSetupCorrectly() {
-    return _currentScheduleSource != null &&
-        !(_currentScheduleSource is InvalidScheduleSource);
+    return !(_currentScheduleSource is InvalidScheduleSource);
   }
 
   void addDidChangeScheduleSourceCallback(OnDidChangeScheduleSource callback) {
diff --git a/lib/schedule/data/schedule_entry_entity.dart b/lib/schedule/data/schedule_entry_entity.dart
index 2f532d8f..8a5ae1bc 100644
--- a/lib/schedule/data/schedule_entry_entity.dart
+++ b/lib/schedule/data/schedule_entry_entity.dart
@@ -2,7 +2,7 @@ import 'package:dhbwstudentapp/common/data/database_entity.dart';
 import 'package:dhbwstudentapp/schedule/model/schedule_entry.dart';
 
 class ScheduleEntryEntity extends DatabaseEntity {
-  ScheduleEntry _scheduleEntry;
+  ScheduleEntry? _scheduleEntry;
 
   ScheduleEntryEntity.fromModel(ScheduleEntry scheduleEntry) {
     _scheduleEntry = scheduleEntry;
@@ -14,15 +14,9 @@ class ScheduleEntryEntity extends DatabaseEntity {
 
   @override
   void fromMap(Map<String, dynamic> map) {
-    DateTime startDate;
-    if (map["start"] != null) {
-      startDate = DateTime.fromMillisecondsSinceEpoch(map["start"]);
-    }
+    var startDate = DateTime.fromMillisecondsSinceEpoch(map["start"]);
 
-    DateTime endDate;
-    if (map["end"] != null) {
-      endDate = DateTime.fromMillisecondsSinceEpoch(map["end"]);
-    }
+    var endDate = DateTime.fromMillisecondsSinceEpoch(map["end"]);
 
     _scheduleEntry = ScheduleEntry(
         id: map["id"],
@@ -38,18 +32,18 @@ class ScheduleEntryEntity extends DatabaseEntity {
   @override
   Map<String, dynamic> toMap() {
     return {
-      "id": _scheduleEntry.id,
-      "start": _scheduleEntry.start?.millisecondsSinceEpoch ?? 0,
-      "end": _scheduleEntry.end?.millisecondsSinceEpoch ?? 0,
-      "details": _scheduleEntry.details ?? "",
-      "professor": _scheduleEntry.professor ?? "",
-      "room": _scheduleEntry.room ?? "",
-      "title": _scheduleEntry.title ?? "",
-      "type": _scheduleEntry.type?.index
+      "id": _scheduleEntry!.id,
+      "start": _scheduleEntry!.start.millisecondsSinceEpoch,
+      "end": _scheduleEntry!.end.millisecondsSinceEpoch,
+      "details": _scheduleEntry!.details,
+      "professor": _scheduleEntry!.professor,
+      "room": _scheduleEntry!.room,
+      "title": _scheduleEntry!.title,
+      "type": _scheduleEntry!.type.index
     };
   }
 
-  ScheduleEntry asScheduleEntry() => _scheduleEntry;
+  ScheduleEntry asScheduleEntry() => _scheduleEntry!;
 
   static String tableName() => "ScheduleEntries";
 }
diff --git a/lib/schedule/data/schedule_entry_repository.dart b/lib/schedule/data/schedule_entry_repository.dart
index 0f72149f..e46ed5db 100644
--- a/lib/schedule/data/schedule_entry_repository.dart
+++ b/lib/schedule/data/schedule_entry_repository.dart
@@ -34,7 +34,7 @@ class ScheduleEntryRepository {
     return schedule;
   }
 
-  Future<ScheduleEntry> queryExistingScheduleEntry(ScheduleEntry entry) async {
+  Future<ScheduleEntry?> queryExistingScheduleEntry(ScheduleEntry entry) async {
     var rows = await _database.queryRows(
       ScheduleEntryEntity.tableName(),
       where: "start=? AND end=? AND title=? AND details=? AND professor=?",
@@ -52,7 +52,7 @@ class ScheduleEntryRepository {
     return ScheduleEntryEntity.fromMap(rows[0]).asScheduleEntry();
   }
 
-  Future<ScheduleEntry> queryNextScheduleEntry(DateTime dateTime) async {
+  Future<ScheduleEntry?> queryNextScheduleEntry(DateTime dateTime) async {
     var nextScheduleEntry = await _database.queryRows(
       ScheduleEntryEntity.tableName(),
       where: "start>?",
@@ -70,18 +70,16 @@ class ScheduleEntryRepository {
     return null;
   }
 
-  Future<List<String>> queryAllNamesOfScheduleEntries() async {
+  Future<List<String?>> queryAllNamesOfScheduleEntries() async {
     var allNames = await _database.rawQuery(
       "SELECT DISTINCT title FROM  ScheduleEntries",
       [],
     );
 
-    return allNames.map((e) => e["title"] as String).toList();
+    return allNames.map((e) => e["title"] as String?).toList();
   }
 
   Future<void> saveScheduleEntry(ScheduleEntry entry) async {
-    var row = ScheduleEntryEntity.fromModel(entry).toMap();
-
     var existingEntry = await queryExistingScheduleEntry(entry);
 
     if (existingEntry != null) {
@@ -89,6 +87,7 @@ class ScheduleEntryRepository {
       return;
     }
 
+    var row = ScheduleEntryEntity.fromModel(entry).toMap();
     if (entry.id == null) {
       var id = await _database.insert(ScheduleEntryEntity.tableName(), row);
       entry.id = id;
@@ -98,7 +97,7 @@ class ScheduleEntryRepository {
   }
 
   Future<void> saveSchedule(Schedule schedule) async {
-    for (var entry in schedule.entries ?? []) {
+    for (var entry in schedule.entries) {
       saveScheduleEntry(entry);
     }
   }
diff --git a/lib/schedule/data/schedule_filter_repository.dart b/lib/schedule/data/schedule_filter_repository.dart
index 39e41480..651bc270 100644
--- a/lib/schedule/data/schedule_filter_repository.dart
+++ b/lib/schedule/data/schedule_filter_repository.dart
@@ -5,14 +5,14 @@ class ScheduleFilterRepository {
 
   ScheduleFilterRepository(this._database);
 
-  Future<List<String>> queryAllHiddenNames() async {
+  Future<List<String?>> queryAllHiddenNames() async {
     var rows = await _database.queryRows("ScheduleEntryFilters");
 
-    var names = rows.map((e) => e['title'] as String).toList();
+    var names = rows.map((e) => e['title'] as String?).toList();
     return names;
   }
 
-  Future<void> saveAllHiddenNames(List<String> hiddenNames) async {
+  Future<void> saveAllHiddenNames(List<String?> hiddenNames) async {
     await _database.deleteWhere("ScheduleEntryFilters");
 
     for (var name in hiddenNames) {
diff --git a/lib/schedule/data/schedule_query_information_entity.dart b/lib/schedule/data/schedule_query_information_entity.dart
index abd0d5e2..b8bc4e1d 100644
--- a/lib/schedule/data/schedule_query_information_entity.dart
+++ b/lib/schedule/data/schedule_query_information_entity.dart
@@ -2,7 +2,7 @@ import 'package:dhbwstudentapp/common/data/database_entity.dart';
 import 'package:dhbwstudentapp/schedule/model/schedule_query_information.dart';
 
 class ScheduleQueryInformationEntity extends DatabaseEntity {
-  ScheduleQueryInformation _scheduleQueryInformation;
+  ScheduleQueryInformation? _scheduleQueryInformation;
 
   ScheduleQueryInformationEntity.fromModel(
     ScheduleQueryInformation scheduleQueryInformation,
@@ -16,18 +16,18 @@ class ScheduleQueryInformationEntity extends DatabaseEntity {
 
   @override
   void fromMap(Map<String, dynamic> map) {
-    DateTime startDate;
+    DateTime? startDate;
     if (map["start"] != null) {
       startDate = DateTime.fromMillisecondsSinceEpoch(map["start"]);
     }
 
-    DateTime endDate;
+    DateTime? endDate;
     if (map["end"] != null) {
       endDate = DateTime.fromMillisecondsSinceEpoch(map["end"]);
     }
 
-    DateTime queryTimeDate;
-    if (map["end"] != null) {
+    DateTime? queryTimeDate;
+    if (map["queryTime"] != null) {
       queryTimeDate = DateTime.fromMillisecondsSinceEpoch(map["queryTime"]);
     }
 
@@ -38,13 +38,13 @@ class ScheduleQueryInformationEntity extends DatabaseEntity {
   @override
   Map<String, dynamic> toMap() {
     return {
-      "start": _scheduleQueryInformation.start?.millisecondsSinceEpoch ?? 0,
-      "end": _scheduleQueryInformation.end?.millisecondsSinceEpoch ?? 0,
-      "queryTime": _scheduleQueryInformation.queryTime?.millisecondsSinceEpoch,
+      "start": _scheduleQueryInformation!.start.millisecondsSinceEpoch,
+      "end": _scheduleQueryInformation!.end.millisecondsSinceEpoch,
+      "queryTime": _scheduleQueryInformation!.queryTime?.millisecondsSinceEpoch,
     };
   }
 
-  ScheduleQueryInformation asScheduleQueryInformation() =>
+  ScheduleQueryInformation? asScheduleQueryInformation() =>
       _scheduleQueryInformation;
 
   static String tableName() => "ScheduleQueryInformation";
diff --git a/lib/schedule/data/schedule_query_information_repository.dart b/lib/schedule/data/schedule_query_information_repository.dart
index 669d300e..e43d8fcf 100644
--- a/lib/schedule/data/schedule_query_information_repository.dart
+++ b/lib/schedule/data/schedule_query_information_repository.dart
@@ -7,7 +7,7 @@ class ScheduleQueryInformationRepository {
 
   ScheduleQueryInformationRepository(this._database);
 
-  Future<DateTime> getOldestQueryTimeBetweenDates(
+  Future<DateTime?> getOldestQueryTimeBetweenDates(
       DateTime start, DateTime end) async {
     var oldestQueryTimeDate = await _database.queryAggregator(
       "SELECT MIN(queryTime) FROM ScheduleQueryInformation WHERE start<=? AND end>=?",
@@ -17,10 +17,12 @@ class ScheduleQueryInformationRepository {
       ],
     );
 
+    if (oldestQueryTimeDate == null) return null;
+
     return DateTime.fromMillisecondsSinceEpoch(oldestQueryTimeDate);
   }
 
-  Future<List<ScheduleQueryInformation>> getQueryInformationBetweenDates(
+  Future<List<ScheduleQueryInformation?>> getQueryInformationBetweenDates(
       DateTime start, DateTime end) async {
     var rows = await _database.queryRows(
       ScheduleQueryInformationEntity.tableName(),
@@ -31,7 +33,7 @@ class ScheduleQueryInformationRepository {
       ],
     );
 
-    var scheduleQueryInformation = <ScheduleQueryInformation>[];
+    var scheduleQueryInformation = <ScheduleQueryInformation?>[];
 
     for (var row in rows) {
       scheduleQueryInformation.add(
diff --git a/lib/schedule/model/schedule.dart b/lib/schedule/model/schedule.dart
index 7a948471..3d4aeac4 100644
--- a/lib/schedule/model/schedule.dart
+++ b/lib/schedule/model/schedule.dart
@@ -25,11 +25,11 @@ class Schedule {
     }
   }
 
-  Schedule trim(DateTime startDate, DateTime endDate) {
+  Schedule trim(DateTime? startDate, DateTime? endDate) {
     var newList = <ScheduleEntry>[];
 
     for (var entry in entries) {
-      if (startDate.isBefore(entry.end) && endDate.isAfter(entry.start)) {
+      if (startDate!.isBefore(entry.end) && endDate!.isAfter(entry.start)) {
         newList.add(entry);
       }
     }
@@ -40,28 +40,28 @@ class Schedule {
     return schedule;
   }
 
-  DateTime getStartDate() {
+  DateTime? getStartDate() {
     if (entries.isEmpty) return null;
 
-    var date = entries?.reduce((ScheduleEntry a, ScheduleEntry b) {
-      return a.start.isBefore(b.start) ? a : b;
-    })?.start;
+    var date = entries.reduce((ScheduleEntry? a, ScheduleEntry? b) {
+      return a!.start.isBefore(b!.start) ? a : b;
+    }).start;
 
     return date;
   }
 
-  DateTime getEndDate() {
+  DateTime? getEndDate() {
     if (entries.isEmpty) return null;
 
-    var date = entries?.reduce((ScheduleEntry a, ScheduleEntry b) {
-      return a.end.isAfter(b.end) ? a : b;
-    })?.end;
+    var date = entries.reduce((ScheduleEntry? a, ScheduleEntry? b) {
+      return a!.end.isAfter(b!.end) ? a : b;
+    }).end;
 
     return date;
   }
 
-  DateTime getStartTime() {
-    DateTime earliestTime;
+  DateTime? getStartTime() {
+    DateTime? earliestTime;
 
     for (var entry in entries) {
       var entryTime = DateTime(
@@ -83,8 +83,8 @@ class Schedule {
     return earliestTime;
   }
 
-  DateTime getEndTime() {
-    DateTime latestTime;
+  DateTime? getEndTime() {
+    DateTime? latestTime;
 
     for (var entry in entries) {
       var entryTime = DateTime(
@@ -106,7 +106,7 @@ class Schedule {
     return latestTime;
   }
 
-  Schedule copyWith({List<ScheduleEntry> entries}) {
+  Schedule copyWith({required List<ScheduleEntry> entries}) {
     var schedule = Schedule.fromList(entries);
 
     schedule.urls.addAll(urls);
diff --git a/lib/schedule/model/schedule_entry.dart b/lib/schedule/model/schedule_entry.dart
index 619df572..d925d5b0 100644
--- a/lib/schedule/model/schedule_entry.dart
+++ b/lib/schedule/model/schedule_entry.dart
@@ -7,7 +7,7 @@ enum ScheduleEntryType {
 }
 
 class ScheduleEntry {
-  int id;
+  int? id;
   final DateTime start;
   final DateTime end;
   final String title;
@@ -18,14 +18,19 @@ class ScheduleEntry {
 
   ScheduleEntry({
     this.id,
-    this.start,
-    this.end,
-    this.title,
-    this.details,
-    this.professor,
-    this.room,
-    this.type,
-  });
+    DateTime? start,
+    DateTime? end,
+    String? title,
+    String? details,
+    String? professor,
+    String? room,
+    required this.type,
+  })  : this.start = start ?? DateTime.fromMicrosecondsSinceEpoch(0),
+        this.end = end ?? DateTime.fromMicrosecondsSinceEpoch(0),
+        this.details = details ?? "",
+        this.professor = professor ?? "",
+        this.room = room ?? "",
+        this.title = title ?? "";
 
   bool equalsWithIdIgnored(ScheduleEntry other) {
     return this.start == other.start &&
@@ -40,7 +45,7 @@ class ScheduleEntry {
   List<String> getDifferentProperties(ScheduleEntry entry) {
     var changedProperties = <String>[];
 
-    if ((title ?? "") != (entry.title ?? "")) {
+    if (title != entry.title) {
       changedProperties.add("title");
     }
     if (start != entry.start) {
@@ -49,13 +54,13 @@ class ScheduleEntry {
     if (end != entry.end) {
       changedProperties.add("end");
     }
-    if ((details ?? "") != (entry.details ?? "")) {
+    if (details != entry.details) {
       changedProperties.add("details");
     }
-    if ((professor ?? "") != (entry.professor ?? "")) {
+    if (professor != entry.professor) {
       changedProperties.add("professor");
     }
-    if ((room ?? "") != (entry.room ?? "")) {
+    if (room != entry.room) {
       changedProperties.add("room");
     }
     if (type != entry.type) {
@@ -65,14 +70,15 @@ class ScheduleEntry {
     return changedProperties;
   }
 
+  // TODO: [Leptopoda] use buildrunner
   ScheduleEntry copyWith(
-      {DateTime start,
-      DateTime end,
-      String title,
-      String details,
-      String professor,
-      String room,
-      ScheduleEntryType type}) {
+      {DateTime? start,
+      DateTime? end,
+      String? title,
+      String? details,
+      String? professor,
+      String? room,
+      ScheduleEntryType? type}) {
     return ScheduleEntry(
       id: id,
       start: start ?? this.start,
diff --git a/lib/schedule/model/schedule_query_information.dart b/lib/schedule/model/schedule_query_information.dart
index e527f01a..b9e124b8 100644
--- a/lib/schedule/model/schedule_query_information.dart
+++ b/lib/schedule/model/schedule_query_information.dart
@@ -1,7 +1,12 @@
 class ScheduleQueryInformation {
   final DateTime start;
   final DateTime end;
-  final DateTime queryTime;
+  final DateTime? queryTime;
 
-  ScheduleQueryInformation(this.start, this.end, this.queryTime);
+  ScheduleQueryInformation(
+    DateTime? start,
+    DateTime? end,
+    this.queryTime,
+  )   : this.start = start ?? DateTime.fromMillisecondsSinceEpoch(0),
+        this.end = end ?? DateTime.fromMillisecondsSinceEpoch(0);
 }
diff --git a/lib/schedule/model/schedule_query_result.dart b/lib/schedule/model/schedule_query_result.dart
index 26382398..421807bc 100644
--- a/lib/schedule/model/schedule_query_result.dart
+++ b/lib/schedule/model/schedule_query_result.dart
@@ -4,16 +4,16 @@ class ScheduleQueryResult {
   final Schedule schedule;
   final List<ParseError> errors;
 
-  bool get hasError => errors?.isNotEmpty ?? false;
+  bool get hasError => errors.isNotEmpty;
 
   ScheduleQueryResult(this.schedule, this.errors);
 }
 
 class ParseError {
   final String object;
-  final String trace;
+  final String? trace;
 
-  ParseError(Object object, StackTrace trace)
-      : object = object?.toString(),
+  ParseError(Object object, [StackTrace? trace])
+      : object = object.toString(),
         trace = trace?.toString();
 }
diff --git a/lib/schedule/service/dualis/dualis_schedule_source.dart b/lib/schedule/service/dualis/dualis_schedule_source.dart
index f515f501..9863689f 100644
--- a/lib/schedule/service/dualis/dualis_schedule_source.dart
+++ b/lib/schedule/service/dualis/dualis_schedule_source.dart
@@ -13,11 +13,11 @@ class DualisScheduleSource extends ScheduleSource {
   DualisScheduleSource(this._dualisScraper);
 
   @override
-  Future<ScheduleQueryResult> querySchedule(DateTime from, DateTime to,
-      [CancellationToken cancellationToken]) async {
+  Future<ScheduleQueryResult> querySchedule(DateTime? from, DateTime? to,
+      [CancellationToken? cancellationToken]) async {
     if (cancellationToken == null) cancellationToken = CancellationToken();
 
-    DateTime current = toStartOfMonth(from);
+    DateTime current = toStartOfMonth(from)!;
 
     var schedule = Schedule();
     var allErrors = <ParseError>[];
@@ -25,12 +25,12 @@ class DualisScheduleSource extends ScheduleSource {
     if (!_dualisScraper.isLoggedIn())
       await _dualisScraper.loginWithPreviousCredentials(cancellationToken);
 
-    while (to.isAfter(current) && !cancellationToken.isCancelled()) {
+    while (to!.isAfter(current) && !cancellationToken.isCancelled()) {
       try {
         var monthSchedule = await _dualisScraper.loadMonthlySchedule(
             current, cancellationToken);
 
-        if (monthSchedule != null) schedule.merge(monthSchedule);
+        schedule.merge(monthSchedule);
       } on OperationCancelledException {
         rethrow;
       } on ParseException catch (ex, trace) {
@@ -51,10 +51,7 @@ class DualisScheduleSource extends ScheduleSource {
   }
 
   Future<void> setLoginCredentials(Credentials credentials) async {
-    _dualisScraper.setLoginCredentials(
-      credentials.username,
-      credentials.password,
-    );
+    _dualisScraper.setLoginCredentials(credentials);
   }
 
   @override
diff --git a/lib/schedule/service/error_report_schedule_source_decorator.dart b/lib/schedule/service/error_report_schedule_source_decorator.dart
index 15c5e88b..d1930b98 100644
--- a/lib/schedule/service/error_report_schedule_source_decorator.dart
+++ b/lib/schedule/service/error_report_schedule_source_decorator.dart
@@ -9,8 +9,8 @@ class ErrorReportScheduleSourceDecorator extends ScheduleSource {
   ErrorReportScheduleSourceDecorator(this._scheduleSource);
 
   @override
-  Future<ScheduleQueryResult> querySchedule(DateTime from, DateTime to,
-      [CancellationToken cancellationToken]) async {
+  Future<ScheduleQueryResult?> querySchedule(DateTime? from, DateTime? to,
+      [CancellationToken? cancellationToken]) async {
     try {
       var schedule = await _scheduleSource.querySchedule(
         from,
diff --git a/lib/schedule/service/ical/ical_parser.dart b/lib/schedule/service/ical/ical_parser.dart
index 60435489..5042a245 100644
--- a/lib/schedule/service/ical/ical_parser.dart
+++ b/lib/schedule/service/ical/ical_parser.dart
@@ -34,7 +34,7 @@ class IcalParser {
     List<ScheduleEntry> entries = [];
 
     for (var match in matches) {
-      var entry = _parseEntry(match.group(1));
+      var entry = _parseEntry(match.group(1)!);
       entries.add(entry);
     }
 
@@ -50,7 +50,7 @@ class IcalParser {
       unicode: true,
     ).allMatches(entryData);
 
-    Map<String, String> properties = {};
+    Map<String?, String?> properties = {};
 
     for (var property in allProperties) {
       properties[property.group(1)] = property.group(3);
@@ -67,23 +67,23 @@ class IcalParser {
     );
   }
 
-  DateTime _parseDate(String date) {
+  DateTime? _parseDate(String? date) {
+    if (date == null) return null;
+
     var match = RegExp(
       dateTimeRegex,
       unicode: true,
-    ).firstMatch(date ?? "");
+    ).firstMatch(date);
 
-    if (match == null) {
-      return null;
-    }
+    if (match == null) return null;
 
     return DateTime(
-      int.tryParse(match.group(1)),
-      int.tryParse(match.group(2)),
-      int.tryParse(match.group(3)),
-      int.tryParse(match.group(4)),
-      int.tryParse(match.group(5)),
-      int.tryParse(match.group(6)),
+      int.tryParse(match.group(1)!)!,
+      int.tryParse(match.group(2)!)!,
+      int.tryParse(match.group(3)!)!,
+      int.tryParse(match.group(4)!)!,
+      int.tryParse(match.group(5)!)!,
+      int.tryParse(match.group(6)!)!,
     );
   }
 }
diff --git a/lib/schedule/service/ical/ical_schedule_source.dart b/lib/schedule/service/ical/ical_schedule_source.dart
index 495ada39..2bf3112a 100644
--- a/lib/schedule/service/ical/ical_schedule_source.dart
+++ b/lib/schedule/service/ical/ical_schedule_source.dart
@@ -10,9 +10,9 @@ import 'package:http_client_helper/http_client_helper.dart' as http;
 
 class IcalScheduleSource extends ScheduleSource {
   final IcalParser _icalParser = IcalParser();
-  String _url;
+  String? _url;
 
-  void setIcalUrl(String url) {
+  void setIcalUrl(String? url) {
     _url = url;
   }
 
@@ -22,12 +22,12 @@ class IcalScheduleSource extends ScheduleSource {
   }
 
   @override
-  Future<ScheduleQueryResult> querySchedule(
-    DateTime from,
-    DateTime to, [
-    CancellationToken cancellationToken,
+  Future<ScheduleQueryResult?> querySchedule(
+    DateTime? from,
+    DateTime? to, [
+    CancellationToken? cancellationToken,
   ]) async {
-    var response = await _makeRequest(_url, cancellationToken);
+    var response = await _makeRequest(_url!, cancellationToken!);
     if (response == null) return null;
 
     try {
@@ -45,7 +45,7 @@ class IcalScheduleSource extends ScheduleSource {
     }
   }
 
-  Future<Response> _makeRequest(
+  Future<Response?> _makeRequest(
       String url, CancellationToken cancellationToken) async {
     url = url.replaceAll("webcal://", "https://");
 
@@ -76,8 +76,9 @@ class IcalScheduleSource extends ScheduleSource {
     return null;
   }
 
-  static bool isValidUrl(String url) {
+  static bool isValidUrl(String? url) {
     try {
+      if (url == null) return false;
       Uri.parse(url);
     } catch (e) {
       return false;
diff --git a/lib/schedule/service/invalid_schedule_source.dart b/lib/schedule/service/invalid_schedule_source.dart
index 0dcd0030..7f55936b 100644
--- a/lib/schedule/service/invalid_schedule_source.dart
+++ b/lib/schedule/service/invalid_schedule_source.dart
@@ -5,9 +5,9 @@ import 'package:dhbwstudentapp/schedule/service/schedule_source.dart';
 class InvalidScheduleSource extends ScheduleSource {
   @override
   Future<ScheduleQueryResult> querySchedule(
-    DateTime from,
-    DateTime to, [
-    CancellationToken cancellationToken,
+    DateTime? from,
+    DateTime? to, [
+    CancellationToken? cancellationToken,
   ]) {
     throw StateError("Schedule source not properly configured");
   }
diff --git a/lib/schedule/service/isolate_schedule_source_decorator.dart b/lib/schedule/service/isolate_schedule_source_decorator.dart
index 02ea3a96..5071aed6 100644
--- a/lib/schedule/service/isolate_schedule_source_decorator.dart
+++ b/lib/schedule/service/isolate_schedule_source_decorator.dart
@@ -11,35 +11,35 @@ import 'package:dhbwstudentapp/schedule/service/schedule_source.dart';
 class IsolateScheduleSourceDecorator extends ScheduleSource {
   final ScheduleSource _scheduleSource;
 
-  Stream _isolateToMain;
-  Isolate _isolate;
-  SendPort _sendPort;
+  Stream? _isolateToMain;
+  Isolate? _isolate;
+  SendPort? _sendPort;
 
   IsolateScheduleSourceDecorator(this._scheduleSource);
 
   @override
-  Future<ScheduleQueryResult> querySchedule(DateTime from, DateTime to,
-      [CancellationToken cancellationToken]) async {
+  Future<ScheduleQueryResult?> querySchedule(DateTime? from, DateTime? to,
+      [CancellationToken? cancellationToken]) async {
     await _initializeIsolate();
 
     // Use the cancellation token to send a cancel message.
     // The isolate then uses a new instance to cancel the request
-    cancellationToken.setCancellationCallback(() {
-      _sendPort.send({"type": "cancel"});
+    cancellationToken!.setCancellationCallback(() {
+      _sendPort!.send({"type": "cancel"});
     });
 
-    _sendPort.send({
+    _sendPort!.send({
       "type": "execute",
       "source": _scheduleSource,
       "from": from,
       "to": to,
     });
 
-    final completer = Completer<ScheduleQueryResult>();
+    final completer = Completer<ScheduleQueryResult?>();
 
-    ScheduleQueryFailedException potentialException;
+    ScheduleQueryFailedException? potentialException;
 
-    final subscription = _isolateToMain.listen((result) {
+    final subscription = _isolateToMain!.listen((result) {
       cancellationToken.setCancellationCallback(null);
 
       if (result != null && !(result is ScheduleQueryResult)) {
@@ -55,7 +55,7 @@ class IsolateScheduleSourceDecorator extends ScheduleSource {
     subscription.cancel();
 
     if (potentialException != null) {
-      throw potentialException;
+      throw potentialException!;
     }
 
     return result;
@@ -70,7 +70,7 @@ class IsolateScheduleSourceDecorator extends ScheduleSource {
     _isolateToMain = isolateToMain.asBroadcastStream();
     _isolate = await Isolate.spawn(
         scheduleSourceIsolateEntryPoint, isolateToMain.sendPort);
-    _sendPort = await _isolateToMain.first;
+    _sendPort = await _isolateToMain!.first as SendPort?;
   }
 
   @override
@@ -84,7 +84,7 @@ void scheduleSourceIsolateEntryPoint(SendPort sendPort) async {
   var port = ReceivePort();
   sendPort.send(port.sendPort);
 
-  CancellationToken token;
+  CancellationToken? token;
 
   await for (var message in port) {
     if (message["type"] == "execute") {
@@ -99,12 +99,12 @@ void scheduleSourceIsolateEntryPoint(SendPort sendPort) async {
 Future<void> executeQueryScheduleMessage(
   Map<String, dynamic> map,
   SendPort sendPort,
-  CancellationToken token,
+  CancellationToken? token,
 ) async {
   try {
     ScheduleSource source = map["source"];
-    DateTime from = map["from"];
-    DateTime to = map["to"];
+    DateTime? from = map["from"];
+    DateTime? to = map["to"];
 
     var result = await source.querySchedule(from, to, token);
 
diff --git a/lib/schedule/service/mannheim/mannheim_course_response_parser.dart b/lib/schedule/service/mannheim/mannheim_course_response_parser.dart
index 0cb543f0..9912082b 100644
--- a/lib/schedule/service/mannheim/mannheim_course_response_parser.dart
+++ b/lib/schedule/service/mannheim/mannheim_course_response_parser.dart
@@ -14,7 +14,7 @@ class MannheimCourseResponseParser {
     for (var e in options) {
       var label = e.attributes["label"] ?? "";
       var value = e.attributes["value"] ?? "";
-      var title = e.parent.attributes["label"] ?? "";
+      var title = e.parent!.attributes["label"] ?? "";
 
       if (label == "" || value == "") continue;
       if (label.trim() == "Kurs auswählen") continue;
diff --git a/lib/schedule/service/mannheim/mannheim_course_scraper.dart b/lib/schedule/service/mannheim/mannheim_course_scraper.dart
index 7ca27aa0..ecd3b732 100644
--- a/lib/schedule/service/mannheim/mannheim_course_scraper.dart
+++ b/lib/schedule/service/mannheim/mannheim_course_scraper.dart
@@ -15,7 +15,7 @@ class Course {
 
 class MannheimCourseScraper {
   Future<List<Course>> loadCourses([
-    CancellationToken cancellationToken,
+    CancellationToken? cancellationToken,
   ]) async {
     if (cancellationToken == null) cancellationToken = CancellationToken();
 
@@ -42,15 +42,14 @@ class MannheimCourseScraper {
       if (response == null && !requestCancellationToken.isCanceled)
         throw ServiceRequestFailed("Http request failed!");
 
-      return response;
+      return response!;
     } on http.OperationCanceledError catch (_) {
       throw OperationCancelledException();
     } catch (ex) {
       if (!requestCancellationToken.isCanceled) rethrow;
+      throw ServiceRequestFailed("Http request failed!");
     } finally {
       cancellationToken.setCancellationCallback(null);
     }
-
-    return null;
   }
 }
diff --git a/lib/schedule/service/rapla/rapla_monthly_response_parser.dart b/lib/schedule/service/rapla/rapla_monthly_response_parser.dart
index 5886f610..2a2f6bd3 100644
--- a/lib/schedule/service/rapla/rapla_monthly_response_parser.dart
+++ b/lib/schedule/service/rapla/rapla_monthly_response_parser.dart
@@ -12,7 +12,7 @@ class RaplaMonthlyResponseParser {
   static ScheduleQueryResult parseMonthlyTable(
     Element monthTable,
   ) {
-    var title = monthTable.parent.getElementsByClassName("title");
+    var title = monthTable.parent!.getElementsByClassName("title");
 
     var monthAndYear = title[0].text;
     var yearString = monthAndYear.split(" ")[1];
@@ -37,7 +37,7 @@ class RaplaMonthlyResponseParser {
         try {
           var entry = RaplaParsingUtils.extractScheduleEntryOrThrow(
             dayEntry,
-            DateTime(year, month, day),
+            DateTime(year!, month!, day!),
           );
 
           allEntries.add(entry);
@@ -50,7 +50,7 @@ class RaplaMonthlyResponseParser {
     return ScheduleQueryResult(Schedule.fromList(allEntries), parseErrors);
   }
 
-  static int _monthStringToDateTime(String monthAndYear) {
+  static int? _monthStringToDateTime(String monthAndYear) {
     var monthString = monthAndYear.split(" ")[0];
     var monthNames = {
       "Januar": DateTime.january,
diff --git a/lib/schedule/service/rapla/rapla_parsing_utils.dart b/lib/schedule/service/rapla/rapla_parsing_utils.dart
index 90cc3aec..51830201 100644
--- a/lib/schedule/service/rapla/rapla_parsing_utils.dart
+++ b/lib/schedule/service/rapla/rapla_parsing_utils.dart
@@ -46,7 +46,7 @@ class RaplaParsingUtils {
     if (start == null || end == null)
       throw ElementNotFoundParseException("start and end date container");
 
-    ScheduleEntry scheduleEntry;
+    ScheduleEntry? scheduleEntry;
 
     // The important information is stored in a html element called tooltip.
     // Depending on the Rapla configuration the tooltip is available or not.
@@ -66,12 +66,12 @@ class RaplaParsingUtils {
   }
 
   static ScheduleEntry improveScheduleEntry(ScheduleEntry scheduleEntry) {
-    if (scheduleEntry.title == null) {
+    if (scheduleEntry.title == "") {
       throw ElementNotFoundParseException("title");
     }
 
     var professor = scheduleEntry.professor;
-    if (professor?.endsWith(",") ?? false) {
+    if (professor.endsWith(",")) {
       scheduleEntry = scheduleEntry.copyWith(
           professor: professor.substring(0, professor.length - 1));
     }
@@ -87,7 +87,7 @@ class RaplaParsingUtils {
   static ScheduleEntry extractScheduleFromTooltip(
       List<Element> tooltip,
       Element value,
-      ScheduleEntry scheduleEntry,
+      ScheduleEntry? scheduleEntry,
       DateTime start,
       DateTime end) {
     var infotable = tooltip[0].getElementsByClassName(INFOTABLE_CLASS);
@@ -120,7 +120,7 @@ class RaplaParsingUtils {
 
   static ScheduleEntry extractScheduleDetailsFromCell(
       List<Element> timeAndClassName,
-      ScheduleEntry scheduleEntry,
+      ScheduleEntry? scheduleEntry,
       DateTime start,
       DateTime end) {
     var descriptionHtml = timeAndClassName[0].innerHtml.substring(12);
@@ -156,12 +156,11 @@ class RaplaParsingUtils {
 
     var typeString = strongTag[0].innerHtml;
 
-    var type = ScheduleEntryType.Unknown;
     if (entryTypeMapping.containsKey(typeString)) {
-      type = entryTypeMapping[typeString];
+      return entryTypeMapping[typeString]!;
+    } else {
+      return ScheduleEntryType.Unknown;
     }
-
-    return type;
   }
 
   static Map<String, String> _parsePropertiesTable(Element infotable) {
@@ -175,7 +174,7 @@ class RaplaParsingUtils {
     return map;
   }
 
-  static DateTime _parseTime(String timeString, DateTime date) {
+  static DateTime? _parseTime(String timeString, DateTime date) {
     try {
       var time = DateFormat("HH:mm").parse(timeString.substring(0, 5));
       return DateTime(date.year, date.month, date.day, time.hour, time.minute);
@@ -200,7 +199,7 @@ class RaplaParsingUtils {
     // selected year in the date selector
     var comboBoxes = document.getElementsByTagName("select");
 
-    String year;
+    String? year;
     for (var box in comboBoxes) {
       if (box.attributes.containsKey("name") &&
           box.attributes["name"] == "year") {
diff --git a/lib/schedule/service/rapla/rapla_response_parser.dart b/lib/schedule/service/rapla/rapla_response_parser.dart
index f0a7e353..cc37efff 100644
--- a/lib/schedule/service/rapla/rapla_response_parser.dart
+++ b/lib/schedule/service/rapla/rapla_response_parser.dart
@@ -27,7 +27,7 @@ class RaplaResponseParser {
     }
 
     return ScheduleQueryResult(Schedule(), [
-      ParseError("Did not find a week_table and month_table class", null),
+      ParseError("Did not find a week_table and month_table class"),
     ]);
   }
 }
diff --git a/lib/schedule/service/rapla/rapla_schedule_source.dart b/lib/schedule/service/rapla/rapla_schedule_source.dart
index fac25cf4..02082dbe 100644
--- a/lib/schedule/service/rapla/rapla_schedule_source.dart
+++ b/lib/schedule/service/rapla/rapla_schedule_source.dart
@@ -11,7 +11,7 @@ import 'package:http_client_helper/http_client_helper.dart' as http;
 class RaplaScheduleSource extends ScheduleSource {
   final RaplaResponseParser responseParser = RaplaResponseParser();
 
-  String raplaUrl;
+  String? raplaUrl;
 
   RaplaScheduleSource({this.raplaUrl});
 
@@ -20,9 +20,9 @@ class RaplaScheduleSource extends ScheduleSource {
   }
 
   @override
-  Future<ScheduleQueryResult> querySchedule(DateTime from, DateTime to,
-      [CancellationToken cancellationToken]) async {
-    DateTime current = toDayOfWeek(from, DateTime.monday);
+  Future<ScheduleQueryResult> querySchedule(DateTime? from, DateTime? to,
+      [CancellationToken? cancellationToken]) async {
+    DateTime current = toDayOfWeek(from, DateTime.monday)!;
 
     if (cancellationToken == null) cancellationToken = CancellationToken();
 
@@ -31,16 +31,18 @@ class RaplaScheduleSource extends ScheduleSource {
 
     var didChangeMonth = false;
 
-    while ((to.isAfter(current) && !cancellationToken.isCancelled()) ||
+    while ((to!.isAfter(current) && !cancellationToken.isCancelled()) ||
         didChangeMonth) {
       try {
         var weekSchedule = await _fetchRaplaSource(current, cancellationToken);
 
-        if (weekSchedule.schedule != null) {
-          schedule.merge(weekSchedule.schedule);
+        if (weekSchedule?.schedule != null) {
+          schedule.merge(weekSchedule!.schedule);
         }
 
-        allErrors.addAll(weekSchedule.errors ?? []);
+        if (weekSchedule != null) {
+          allErrors.addAll(weekSchedule.errors);
+        }
       } on OperationCancelledException {
         rethrow;
       } on ParseException catch (ex, trace) {
@@ -50,7 +52,7 @@ class RaplaScheduleSource extends ScheduleSource {
       }
 
       var currentMonth = current.month;
-      current = toNextWeek(current);
+      current = toNextWeek(current)!;
       var nextMonth = current.month;
 
       // Some rapla instances only return the dates in the current month.
@@ -67,7 +69,7 @@ class RaplaScheduleSource extends ScheduleSource {
     return ScheduleQueryResult(schedule, allErrors);
   }
 
-  Future<ScheduleQueryResult> _fetchRaplaSource(
+  Future<ScheduleQueryResult?> _fetchRaplaSource(
     DateTime date,
     CancellationToken cancellationToken,
   ) async {
@@ -79,9 +81,7 @@ class RaplaScheduleSource extends ScheduleSource {
     try {
       var schedule = responseParser.parseSchedule(response.body);
 
-      if (schedule?.schedule?.urls != null) {
-        schedule.schedule.urls.add(requestUri.toString());
-      }
+      schedule.schedule.urls.add(requestUri.toString());
 
       return schedule;
     } on ParseException catch (_) {
@@ -100,11 +100,11 @@ class RaplaScheduleSource extends ScheduleSource {
   /// - <rapla_url>?user=XXXXXXXXXX&file=XXXXX&page=XXXXXX
   ///
   Uri _buildRequestUri(DateTime date) {
-    if (!raplaUrl.startsWith("http://") && !raplaUrl.startsWith("https://")) {
+    if (!raplaUrl!.startsWith("http://") && !raplaUrl!.startsWith("https://")) {
       raplaUrl = "http://$raplaUrl";
     }
 
-    var uri = Uri.parse(raplaUrl);
+    var uri = Uri.parse(raplaUrl!);
 
     bool hasKeyParameter = uri.queryParameters.containsKey("key");
     bool hasUserParameter = uri.queryParameters.containsKey("user");
@@ -114,7 +114,7 @@ class RaplaScheduleSource extends ScheduleSource {
     bool hasAllocatableId = uri.queryParameters.containsKey("allocatable_id");
     bool hasSalt = uri.queryParameters.containsKey("salt");
 
-    Map<String, String> parameters = {};
+    Map<String, String?> parameters = {};
 
     if (hasKeyParameter) {
       parameters["key"] = uri.queryParameters["key"];
@@ -133,14 +133,14 @@ class RaplaScheduleSource extends ScheduleSource {
     parameters["month"] = date.month.toString();
     parameters["year"] = date.year.toString();
 
-    if (raplaUrl.startsWith("https")) {
+    if (raplaUrl!.startsWith("https")) {
       return Uri.https(uri.authority, uri.path, parameters);
     } else {
       return Uri.http(uri.authority, uri.path, parameters);
     }
   }
 
-  Future<Response> _makeRequest(
+  Future<Response?> _makeRequest(
       Uri uri, CancellationToken cancellationToken) async {
     var requestCancellationToken = http.CancellationToken();
 
@@ -169,7 +169,7 @@ class RaplaScheduleSource extends ScheduleSource {
 
   @override
   bool canQuery() {
-    return isValidUrl(raplaUrl);
+    return isValidUrl(raplaUrl!);
   }
 
   static bool isValidUrl(String url) {
@@ -181,26 +181,24 @@ class RaplaScheduleSource extends ScheduleSource {
       return false;
     }
 
-    if (uri != null) {
-      bool hasKeyParameter = uri.queryParameters.containsKey("key");
-      bool hasUserParameter = uri.queryParameters.containsKey("user");
-      bool hasFileParameter = uri.queryParameters.containsKey("file");
-      bool hasPageParameter = uri.queryParameters.containsKey("page");
+    bool hasKeyParameter = uri.queryParameters.containsKey("key");
+    bool hasUserParameter = uri.queryParameters.containsKey("user");
+    bool hasFileParameter = uri.queryParameters.containsKey("file");
+    bool hasPageParameter = uri.queryParameters.containsKey("page");
 
-      bool hasAllocatableId = uri.queryParameters.containsKey("allocatable_id");
-      bool hasSalt = uri.queryParameters.containsKey("salt");
+    bool hasAllocatableId = uri.queryParameters.containsKey("allocatable_id");
+    bool hasSalt = uri.queryParameters.containsKey("salt");
 
-      if (hasUserParameter && hasFileParameter && hasPageParameter) {
-        return true;
-      }
+    if (hasUserParameter && hasFileParameter && hasPageParameter) {
+      return true;
+    }
 
-      if (hasSalt && hasAllocatableId && hasKeyParameter) {
-        return true;
-      }
+    if (hasSalt && hasAllocatableId && hasKeyParameter) {
+      return true;
+    }
 
-      if (hasKeyParameter) {
-        return true;
-      }
+    if (hasKeyParameter) {
+      return true;
     }
 
     return false;
@@ -215,9 +213,9 @@ enum FailureReason {
 
 class ScheduleOrFailure {
   final FailureReason reason;
-  final Schedule schedule;
-  final Object exception;
-  final StackTrace trace;
+  final Schedule? schedule;
+  final Object? exception;
+  final StackTrace? trace;
 
   bool get success => reason == FailureReason.Success;
 
diff --git a/lib/schedule/service/rapla/rapla_several_months_response_parser.dart b/lib/schedule/service/rapla/rapla_several_months_response_parser.dart
index dad3b018..96bab905 100644
--- a/lib/schedule/service/rapla/rapla_several_months_response_parser.dart
+++ b/lib/schedule/service/rapla/rapla_several_months_response_parser.dart
@@ -9,14 +9,11 @@ class RaplaSeveralMonthsResponseParser {
     Document document,
     List<Element> monthTables,
   ) {
-
     var parseErrors = <ParseError>[];
     var allEntries = <ScheduleEntry>[];
 
-    for(var monthTable in monthTables) {
-      var result = RaplaMonthlyResponseParser.parseMonthlyTable(
-        monthTable
-      );
+    for (var monthTable in monthTables) {
+      var result = RaplaMonthlyResponseParser.parseMonthlyTable(monthTable);
 
       parseErrors.addAll(result.errors);
       allEntries.addAll(result.schedule.entries);
diff --git a/lib/schedule/service/rapla/rapla_week_response_parser.dart b/lib/schedule/service/rapla/rapla_week_response_parser.dart
index 6d27a811..46fae222 100644
--- a/lib/schedule/service/rapla/rapla_week_response_parser.dart
+++ b/lib/schedule/service/rapla/rapla_week_response_parser.dart
@@ -63,7 +63,7 @@ class RaplaWeekResponseParser {
     }
 
     allEntries.sort(
-      (ScheduleEntry e1, ScheduleEntry e2) => e1?.start?.compareTo(e2?.start),
+      (ScheduleEntry e1, ScheduleEntry e2) => e1.start.compareTo(e2.start),
     );
 
     return ScheduleQueryResult(
@@ -100,7 +100,7 @@ class RaplaWeekResponseParser {
     // selected year in the date selector
     var comboBoxes = document.getElementsByTagName("select");
 
-    String year;
+    String? year;
     for (var box in comboBoxes) {
       if (box.attributes.containsKey("name") &&
           box.attributes["name"] == "year") {
diff --git a/lib/schedule/service/schedule_prettifier.dart b/lib/schedule/service/schedule_prettifier.dart
index a5f9d00e..258bd9cf 100644
--- a/lib/schedule/service/schedule_prettifier.dart
+++ b/lib/schedule/service/schedule_prettifier.dart
@@ -2,8 +2,10 @@ import 'package:dhbwstudentapp/schedule/model/schedule.dart';
 import 'package:dhbwstudentapp/schedule/model/schedule_entry.dart';
 
 class SchedulePrettifier {
-  final RegExp onlinePrefixRegExp = RegExp(r'\(?online\)?([ -]*)', caseSensitive: false);
-  final RegExp onlineSuffixRegExp = RegExp(r'([ -]*)\(?online\)?', caseSensitive: false);
+  final RegExp onlinePrefixRegExp =
+      RegExp(r'\(?online\)?([ -]*)', caseSensitive: false);
+  final RegExp onlineSuffixRegExp =
+      RegExp(r'([ -]*)\(?online\)?', caseSensitive: false);
 
   Schedule prettifySchedule(Schedule schedule) {
     var allEntries = <ScheduleEntry>[];
@@ -41,20 +43,18 @@ class SchedulePrettifier {
   }
 
   ScheduleEntry _removeCourseFromTitle(ScheduleEntry entry) {
-    var title = entry.title ?? "";
-    var details = entry.details ?? "";
+    var title = entry.title;
+    var details = entry.details;
 
     var titleRegex =
-    RegExp("[A-Z]{3,}-?[A-Z]+[0-9]*[A-Z]*[0-9]*[\/]?[A-Z]*[0-9]*[ ]*-?");
+        RegExp("[A-Z]{3,}-?[A-Z]+[0-9]*[A-Z]*[0-9]*[\/]?[A-Z]*[0-9]*[ ]*-?");
     var match = titleRegex.firstMatch(entry.title);
 
     if (match != null && match.start == 0) {
       details = title.substring(0, match.end) + " - $details";
       title = title.substring(match.end).trim();
     } else {
-      var first = title
-          .split(" ")
-          .first;
+      var first = title.split(" ").first;
 
       // Prettify titles: T3MB9025 Fluidmechanik -> Fluidmechanik
 
@@ -62,9 +62,7 @@ class SchedulePrettifier {
       // or less than 2 charcters long
       if (!(first == first.toUpperCase() && first.length >= 3)) return entry;
 
-      var numberCount = first
-          .split(new RegExp("[0-9]"))
-          .length;
+      var numberCount = first.split(new RegExp("[0-9]")).length;
 
       // If there are less thant two numbers in the title, do not prettify it
       if (numberCount < 2) return entry;
diff --git a/lib/schedule/service/schedule_source.dart b/lib/schedule/service/schedule_source.dart
index 87f08c7c..6bd044b3 100644
--- a/lib/schedule/service/schedule_source.dart
+++ b/lib/schedule/service/schedule_source.dart
@@ -9,21 +9,21 @@ abstract class ScheduleSource {
   /// Returns a future which gives the updated schedule or throws an exception
   /// if an error happened or the operation was cancelled
   ///
-  Future<ScheduleQueryResult> querySchedule(DateTime from, DateTime to,
-      [CancellationToken cancellationToken]);
+  Future<ScheduleQueryResult?> querySchedule(DateTime? from, DateTime? to,
+      [CancellationToken? cancellationToken]);
 
   bool canQuery();
 }
 
 class ScheduleQueryFailedException implements Exception {
-  final dynamic innerException;
-  final StackTrace trace;
+  final Object innerException;
+  final StackTrace? trace;
 
   ScheduleQueryFailedException(this.innerException, [this.trace]);
 
   @override
   String toString() {
-    return (innerException?.toString() ?? "") + "\n" + trace?.toString();
+    return innerException.toString() + "\n" + (trace?.toString() ?? "");
   }
 }
 
diff --git a/lib/schedule/ui/dailyschedule/daily_schedule_page.dart b/lib/schedule/ui/dailyschedule/daily_schedule_page.dart
index 8f4342e2..b0f19659 100644
--- a/lib/schedule/ui/dailyschedule/daily_schedule_page.dart
+++ b/lib/schedule/ui/dailyschedule/daily_schedule_page.dart
@@ -1,5 +1,4 @@
 import 'package:dhbwstudentapp/common/i18n/localizations.dart';
-import 'package:dhbwstudentapp/common/ui/viewmodels/base_view_model.dart';
 import 'package:dhbwstudentapp/common/ui/text_styles.dart';
 import 'package:dhbwstudentapp/schedule/ui/dailyschedule/widgets/current_time_indicator_widget.dart';
 import 'package:dhbwstudentapp/schedule/ui/dailyschedule/widgets/daily_schedule_entry_widget.dart';
@@ -11,16 +10,17 @@ import 'package:property_change_notifier/property_change_notifier.dart';
 import 'package:provider/provider.dart';
 
 class DailySchedulePage extends StatefulWidget {
+  const DailySchedulePage({Key? key}) : super(key: key);
   @override
   _DailySchedulePageState createState() => _DailySchedulePageState();
 }
 
 class _DailySchedulePageState extends State<DailySchedulePage> {
-  DailyScheduleViewModel viewModel;
+  late DailyScheduleViewModel viewModel;
 
   @override
   Widget build(BuildContext context) {
-    viewModel = Provider.of<BaseViewModel>(context);
+    viewModel = Provider.of<DailyScheduleViewModel>(context);
 
     return PropertyChangeProvider<DailyScheduleViewModel, String>(
       value: viewModel,
@@ -34,18 +34,18 @@ class _DailySchedulePageState extends State<DailySchedulePage> {
               Padding(
                 padding: const EdgeInsets.fromLTRB(0, 0, 0, 16),
                 child: PropertyChangeConsumer<DailyScheduleViewModel, String>(
-                  builder: (BuildContext context, DailyScheduleViewModel model,
-                      Set properties) {
+                  builder: (BuildContext context, DailyScheduleViewModel? model,
+                      Set? properties) {
                     var dateFormat = DateFormat.yMMMMEEEEd(
                         L.of(context).locale.languageCode);
                     return Text(
-                      dateFormat.format(model.currentDate),
+                      dateFormat.format(model!.currentDate!),
                       style: textStyleDailyScheduleCurrentDate(context),
                     );
                   },
                 ),
               ),
-              (viewModel.daySchedule?.entries?.length ?? 0) == 0
+              viewModel.daySchedule.entries.isEmpty
                   ? Padding(
                       padding: const EdgeInsets.fromLTRB(0, 32, 0, 0),
                       child: Column(
@@ -93,7 +93,7 @@ class _DailySchedulePageState extends State<DailySchedulePage> {
     var now = DateTime.now();
     var nowIndicatorInserted = false;
 
-    for (var entry in viewModel.daySchedule?.entries) {
+    for (var entry in viewModel.daySchedule.entries) {
       if (!nowIndicatorInserted && (entry.end.isAfter(now))) {
         entryWidgets.add(CurrentTimeIndicatorWidget());
         nowIndicatorInserted = true;
diff --git a/lib/schedule/ui/dailyschedule/widgets/daily_schedule_entry_widget.dart b/lib/schedule/ui/dailyschedule/widgets/daily_schedule_entry_widget.dart
index 9c0fe1e1..474c02cd 100644
--- a/lib/schedule/ui/dailyschedule/widgets/daily_schedule_entry_widget.dart
+++ b/lib/schedule/ui/dailyschedule/widgets/daily_schedule_entry_widget.dart
@@ -9,13 +9,11 @@ import 'package:intl/intl.dart';
 class DailyScheduleEntryWidget extends StatelessWidget {
   final ScheduleEntry scheduleEntry;
 
-  const DailyScheduleEntryWidget({Key key, this.scheduleEntry})
+  const DailyScheduleEntryWidget({Key? key, required this.scheduleEntry})
       : super(key: key);
 
   @override
   Widget build(BuildContext context) {
-    if (scheduleEntry == null) return Container();
-
     var timeFormatter = DateFormat.Hm(L.of(context).locale.languageCode);
 
     var startTime = timeFormatter.format(scheduleEntry.start);
@@ -96,7 +94,7 @@ class DailyScheduleEntryWidget extends StatelessWidget {
                           child: Padding(
                             padding: const EdgeInsets.fromLTRB(32, 0, 0, 0),
                             child: Text(
-                              scheduleEntry.room ?? "",
+                              scheduleEntry.room,
                               softWrap: true,
                               textAlign: TextAlign.end,
                             ),
diff --git a/lib/schedule/ui/notification/next_day_information_notification.dart b/lib/schedule/ui/notification/next_day_information_notification.dart
index d70ad2ce..d477ae87 100644
--- a/lib/schedule/ui/notification/next_day_information_notification.dart
+++ b/lib/schedule/ui/notification/next_day_information_notification.dart
@@ -40,8 +40,8 @@ class NextDayInformationNotification extends TaskCallback {
     if (nextScheduleEntry == null) return;
 
     var format = DateFormat.Hm();
-    var daysToNextEntry = toStartOfDay(nextScheduleEntry.start)
-        .difference(toStartOfDay(now))
+    var daysToNextEntry = toStartOfDay(nextScheduleEntry.start)!
+        .difference(toStartOfDay(now)!)
         .inDays;
 
     if (daysToNextEntry > 1) return;
@@ -58,27 +58,30 @@ class NextDayInformationNotification extends TaskCallback {
     );
   }
 
-  String _getNotificationMessage(
+  String? _getNotificationMessage(
       int daysToNextEntry, ScheduleEntry nextScheduleEntry, DateFormat format) {
-    String message;
-    if (daysToNextEntry == 0) {
-      message = interpolate(
-        _localization.notificationNextClassNextClassAtMessage,
-        [
-          nextScheduleEntry.title,
-          format.format(nextScheduleEntry.start),
-        ],
-      );
-    } else if (daysToNextEntry == 1) {
-      message = interpolate(
-        _localization.notificationNextClassTomorrow,
-        [
-          nextScheduleEntry.title,
-          format.format(nextScheduleEntry.start),
-        ],
-      );
+    switch (daysToNextEntry) {
+      case 0:
+        return interpolate(
+          _localization.notificationNextClassNextClassAtMessage,
+          [
+            nextScheduleEntry.title,
+            format.format(nextScheduleEntry.start),
+          ],
+        );
+
+      case 1:
+        return interpolate(
+          _localization.notificationNextClassTomorrow,
+          [
+            nextScheduleEntry.title,
+            format.format(nextScheduleEntry.start),
+          ],
+        );
+
+      default:
+        return null;
     }
-    return message;
   }
 
   @override
diff --git a/lib/schedule/ui/schedule_navigation_entry.dart b/lib/schedule/ui/schedule_navigation_entry.dart
index 096dc47f..bfb542a7 100644
--- a/lib/schedule/ui/schedule_navigation_entry.dart
+++ b/lib/schedule/ui/schedule_navigation_entry.dart
@@ -1,5 +1,4 @@
 import 'package:dhbwstudentapp/common/i18n/localizations.dart';
-import 'package:dhbwstudentapp/common/ui/viewmodels/base_view_model.dart';
 import 'package:dhbwstudentapp/schedule/ui/schedule_page.dart';
 import 'package:dhbwstudentapp/schedule/ui/viewmodels/schedule_view_model.dart';
 import 'package:dhbwstudentapp/schedule/ui/weeklyschedule/filter/schedule_filter_page.dart';
@@ -9,23 +8,15 @@ import 'package:flutter/material.dart';
 import 'package:kiwi/kiwi.dart';
 import 'package:property_change_notifier/property_change_notifier.dart';
 
-class ScheduleNavigationEntry extends NavigationEntry {
-  ScheduleViewModel _viewModel;
-
+class ScheduleNavigationEntry extends NavigationEntry<ScheduleViewModel> {
   @override
-  Widget icon(BuildContext context) {
-    return Icon(Icons.calendar_today);
-  }
+  Icon icon = Icon(Icons.calendar_today);
 
   @override
-  BaseViewModel initViewModel() {
-    if (_viewModel != null) return _viewModel;
-
-    _viewModel = ScheduleViewModel(
+  ScheduleViewModel initViewModel() {
+    return ScheduleViewModel(
       KiwiContainer().resolve(),
     );
-
-    return _viewModel;
   }
 
   @override
@@ -40,29 +31,29 @@ class ScheduleNavigationEntry extends NavigationEntry {
 
   @override
   List<Widget> appBarActions(BuildContext context) {
-    initViewModel();
     return [
       PropertyChangeProvider<ScheduleViewModel, String>(
-        value: _viewModel,
+        value: model,
         child: PropertyChangeConsumer(
             properties: const ["didSetupProperly"],
-            builder: (BuildContext _, ScheduleViewModel __, Set<Object> ___) =>
-                _viewModel.didSetupProperly
-                    ? Container()
-                    : IconButton(
-                        icon: Icon(Icons.help_outline),
-                        onPressed: () async {
-                          await ScheduleHelpDialog().show(context);
-                        },
-                        tooltip: L.of(context).helpButtonTooltip,
-                      )),
+            builder:
+                (BuildContext _, ScheduleViewModel? __, Set<Object>? ___) =>
+                    model.didSetupProperly
+                        ? Container()
+                        : IconButton(
+                            icon: Icon(Icons.help_outline),
+                            onPressed: () async {
+                              await ScheduleHelpDialog().show(context);
+                            },
+                            tooltip: L.of(context).helpButtonTooltip,
+                          )),
       ),
       PropertyChangeProvider<ScheduleViewModel, String>(
-        value: _viewModel,
+        value: model,
         child: PropertyChangeConsumer(
           properties: const ["didSetupProperly"],
-          builder: (BuildContext _, ScheduleViewModel __, Set<Object> ___) =>
-              _viewModel.didSetupProperly
+          builder: (BuildContext _, ScheduleViewModel? __, Set<Object>? ___) =>
+              model.didSetupProperly
                   ? IconButton(
                       icon: Icon(Icons.filter_alt),
                       onPressed: () async {
diff --git a/lib/schedule/ui/schedule_page.dart b/lib/schedule/ui/schedule_page.dart
index 3da65712..531a0687 100644
--- a/lib/schedule/ui/schedule_page.dart
+++ b/lib/schedule/ui/schedule_page.dart
@@ -1,5 +1,4 @@
 import 'package:dhbwstudentapp/common/i18n/localizations.dart';
-import 'package:dhbwstudentapp/common/ui/viewmodels/base_view_model.dart';
 import 'package:dhbwstudentapp/schedule/ui/dailyschedule/daily_schedule_page.dart';
 import 'package:dhbwstudentapp/schedule/ui/viewmodels/daily_schedule_view_model.dart';
 import 'package:dhbwstudentapp/schedule/ui/viewmodels/schedule_view_model.dart';
@@ -12,6 +11,7 @@ import 'package:kiwi/kiwi.dart';
 import 'package:provider/provider.dart';
 
 class SchedulePage extends StatefulWidget {
+  const SchedulePage({Key? key}) : super(key: key);
   @override
   _SchedulePageState createState() => _SchedulePageState();
 }
@@ -29,20 +29,20 @@ class _SchedulePageState extends State<SchedulePage> {
 
   @override
   Widget build(BuildContext context) {
-    ScheduleViewModel viewModel = Provider.of<BaseViewModel>(context);
+    ScheduleViewModel viewModel = Provider.of<ScheduleViewModel>(context);
 
     if (!viewModel.didSetupProperly) {
       return ScheduleEmptyState();
     } else {
       return PagerWidget(
-        pages: <PageDefinition>[
-          PageDefinition(
+        pages: [
+          PageDefinition<WeeklyScheduleViewModel>(
             icon: Icon(Icons.view_week),
             text: L.of(context).pageWeekOverviewTitle,
             builder: (_) => WeeklySchedulePage(),
             viewModel: weeklyScheduleViewModel,
           ),
-          PageDefinition(
+          PageDefinition<DailyScheduleViewModel>(
             icon: Icon(Icons.view_day),
             text: L.of(context).pageDayOverviewTitle,
             builder: (_) => DailySchedulePage(),
diff --git a/lib/schedule/ui/viewmodels/daily_schedule_view_model.dart b/lib/schedule/ui/viewmodels/daily_schedule_view_model.dart
index 793b76ab..0183aa0d 100644
--- a/lib/schedule/ui/viewmodels/daily_schedule_view_model.dart
+++ b/lib/schedule/ui/viewmodels/daily_schedule_view_model.dart
@@ -8,9 +8,9 @@ class DailyScheduleViewModel extends BaseViewModel {
 
   final ScheduleProvider scheduleProvider;
 
-  DateTime currentDate;
+  DateTime? currentDate;
 
-  Schedule daySchedule;
+  Schedule? _daySchedule;
 
   DailyScheduleViewModel(this.scheduleProvider) {
     scheduleProvider.addScheduleUpdatedCallback(_scheduleUpdatedCallback);
@@ -18,11 +18,13 @@ class DailyScheduleViewModel extends BaseViewModel {
     loadScheduleForToday();
   }
 
-  Future setSchedule(Schedule schedule) async {
-    daySchedule = schedule;
+  set schedule(Schedule schedule) {
+    _daySchedule = schedule;
     notifyListeners("daySchedule");
   }
 
+  Schedule get daySchedule => _daySchedule ??= Schedule();
+
   Future loadScheduleForToday() async {
     var now = DateTime.now();
     currentDate = toStartOfDay(now);
@@ -35,30 +37,24 @@ class DailyScheduleViewModel extends BaseViewModel {
   }
 
   Future _updateScheduleFromCache() async {
-    setSchedule(
-      await scheduleProvider.getCachedSchedule(
-        currentDate,
-        tomorrow(currentDate),
-      ),
+    schedule = await scheduleProvider.getCachedSchedule(
+      currentDate!,
+      tomorrow(currentDate)!,
     );
   }
 
   Future<void> _scheduleUpdatedCallback(
     Schedule schedule,
-    DateTime start,
-    DateTime end,
+    DateTime? start,
+    DateTime? end,
   ) async {
-    if (schedule == null) return;
-
     start = toStartOfDay(start);
     end = toStartOfDay(tomorrow(end));
 
-    if (!(start.isAfter(currentDate) || end.isBefore(currentDate))) {
-      setSchedule(
-        schedule.trim(
-          toStartOfDay(currentDate),
-          toStartOfDay(tomorrow(currentDate)),
-        ),
+    if (!(start!.isAfter(currentDate!) || end!.isBefore(currentDate!))) {
+      schedule = schedule.trim(
+        toStartOfDay(currentDate),
+        toStartOfDay(tomorrow(currentDate)),
       );
     }
   }
diff --git a/lib/schedule/ui/viewmodels/weekly_schedule_view_model.dart b/lib/schedule/ui/viewmodels/weekly_schedule_view_model.dart
index af98bb16..076eef85 100644
--- a/lib/schedule/ui/viewmodels/weekly_schedule_view_model.dart
+++ b/lib/schedule/ui/viewmodels/weekly_schedule_view_model.dart
@@ -18,11 +18,11 @@ class WeeklyScheduleViewModel extends BaseViewModel {
   final ScheduleProvider scheduleProvider;
   final ScheduleSourceProvider scheduleSourceProvider;
 
-  DateTime currentDateStart;
-  DateTime currentDateEnd;
+  DateTime? currentDateStart;
+  DateTime? currentDateEnd;
 
-  DateTime clippedDateStart;
-  DateTime clippedDateEnd;
+  DateTime? clippedDateStart;
+  DateTime? clippedDateEnd;
 
   bool didUpdateScheduleIntoFuture = true;
 
@@ -33,24 +33,24 @@ class WeeklyScheduleViewModel extends BaseViewModel {
 
   bool get hasQueryErrors => _hasQueryErrors;
 
-  VoidCallback _queryFailedCallback;
+  VoidCallback? _queryFailedCallback;
 
   bool updateFailed = false;
 
   bool isUpdating = false;
-  Schedule weekSchedule;
+  Schedule? weekSchedule;
 
-  String scheduleUrl;
+  String? scheduleUrl;
 
   DateTime get now => DateTime.now();
 
-  Timer _errorResetTimer;
-  Timer _updateNowTimer;
+  Timer? _errorResetTimer;
+  Timer? _updateNowTimer;
 
   final CancelableMutex _updateMutex = CancelableMutex();
 
-  DateTime lastRequestedStart;
-  DateTime lastRequestedEnd;
+  DateTime? lastRequestedStart;
+  DateTime? lastRequestedEnd;
 
   WeeklyScheduleViewModel(
     this.scheduleProvider,
@@ -62,7 +62,7 @@ class WeeklyScheduleViewModel extends BaseViewModel {
   Future<void> _initViewModel() async {
     _setSchedule(
       null,
-      toDayOfWeek(DateTime.now(), DateTime.monday),
+      toDayOfWeek(DateTime.now(), DateTime.monday)!,
       toDayOfWeek(DateTime.now(), DateTime.friday),
     );
 
@@ -80,15 +80,15 @@ class WeeklyScheduleViewModel extends BaseViewModel {
     if (setupSuccess) await updateSchedule(currentDateStart, currentDateEnd);
   }
 
-  void _setSchedule(Schedule schedule, DateTime start, DateTime end) {
+  void _setSchedule(Schedule? schedule, DateTime start, DateTime? end) {
     weekSchedule = schedule;
     didUpdateScheduleIntoFuture = currentDateStart?.isBefore(start) ?? true;
     currentDateStart = start;
     currentDateEnd = end;
 
     if (weekSchedule != null) {
-      var scheduleStart = weekSchedule.getStartDate();
-      var scheduleEnd = weekSchedule.getEndDate();
+      var scheduleStart = weekSchedule!.getStartDate();
+      var scheduleEnd = weekSchedule!.getEndDate();
 
       if (scheduleStart == null && scheduleEnd == null) {
         clippedDateStart = toDayOfWeek(start, DateTime.monday);
@@ -98,7 +98,7 @@ class WeeklyScheduleViewModel extends BaseViewModel {
         clippedDateEnd = toDayOfWeek(scheduleEnd, DateTime.friday);
       }
 
-      if (scheduleEnd?.isAfter(clippedDateEnd) ?? false)
+      if (scheduleEnd?.isAfter(clippedDateEnd!) ?? false)
         clippedDateEnd = scheduleEnd;
 
       displayStartHour = weekSchedule?.getStartTime()?.hour ?? 23;
@@ -136,7 +136,7 @@ class WeeklyScheduleViewModel extends BaseViewModel {
     await updateSchedule(currentDateStart, currentDateEnd);
   }
 
-  Future updateSchedule(DateTime start, DateTime end) async {
+  Future updateSchedule(DateTime? start, DateTime? end) async {
     lastRequestedEnd = end;
     lastRequestedStart = start;
 
@@ -151,8 +151,9 @@ class WeeklyScheduleViewModel extends BaseViewModel {
       isUpdating = true;
       notifyListeners("isUpdating");
 
-      await _doUpdateSchedule(start, end);
-    } catch (_) {} finally {
+      await _doUpdateSchedule(start!, end!);
+    } catch (_) {
+    } finally {
       isUpdating = false;
       _updateMutex.release();
       notifyListeners("isUpdating");
@@ -162,7 +163,7 @@ class WeeklyScheduleViewModel extends BaseViewModel {
   Future _doUpdateSchedule(DateTime start, DateTime end) async {
     print("Refreshing schedule...");
 
-    var cancellationToken = _updateMutex.token;
+    var cancellationToken = _updateMutex.token!;
 
     scheduleUrl = null;
 
@@ -178,7 +179,7 @@ class WeeklyScheduleViewModel extends BaseViewModel {
     cancellationToken.throwIfCancelled();
 
     if (updatedSchedule?.schedule != null) {
-      var schedule = updatedSchedule.schedule;
+      var schedule = updatedSchedule!.schedule;
 
       _setSchedule(schedule, start, end);
 
@@ -202,7 +203,7 @@ class WeeklyScheduleViewModel extends BaseViewModel {
     print("Refreshing done");
   }
 
-  Future<ScheduleQueryResult> _readScheduleFromService(
+  Future<ScheduleQueryResult?> _readScheduleFromService(
     DateTime start,
     DateTime end,
     CancellationToken token,
@@ -213,14 +214,15 @@ class WeeklyScheduleViewModel extends BaseViewModel {
         end,
         token,
       );
-    } on OperationCancelledException {} on ScheduleQueryFailedException {}
+    } on OperationCancelledException {
+    } on ScheduleQueryFailedException {}
 
     return null;
   }
 
   void _cancelErrorInFuture() {
     if (_errorResetTimer != null) {
-      _errorResetTimer.cancel();
+      _errorResetTimer!.cancel();
     }
 
     _errorResetTimer = Timer(
@@ -233,7 +235,7 @@ class WeeklyScheduleViewModel extends BaseViewModel {
   }
 
   void ensureUpdateNowTimerRunning() {
-    if (_updateNowTimer == null || !_updateNowTimer.isActive) {
+    if (_updateNowTimer == null || !_updateNowTimer!.isActive) {
       _updateNowTimer = Timer.periodic(const Duration(minutes: 1), (_) {
         notifyListeners("now");
       });
diff --git a/lib/schedule/ui/weeklyschedule/filter/filter_view_model.dart b/lib/schedule/ui/weeklyschedule/filter/filter_view_model.dart
index d2555c59..29fa0433 100644
--- a/lib/schedule/ui/weeklyschedule/filter/filter_view_model.dart
+++ b/lib/schedule/ui/weeklyschedule/filter/filter_view_model.dart
@@ -21,7 +21,7 @@ class FilterViewModel extends BaseViewModel {
 
     var filteredNames = await _scheduleFilterRepository.queryAllHiddenNames();
 
-    allNames.sort((s1, s2) => s1.compareTo(s2));
+    allNames.sort((s1, s2) => s1!.compareTo(s2!));
 
     filterStates = allNames.map((e) {
       var isFiltered = filteredNames.contains(e);
@@ -33,7 +33,7 @@ class FilterViewModel extends BaseViewModel {
 
   void applyFilter() async {
     var allFilteredNames = filterStates
-        .where((element) => !element.isDisplayed)
+        .where((element) => !element.isDisplayed!)
         .map((e) => e.entryName)
         .toList();
 
@@ -44,8 +44,8 @@ class FilterViewModel extends BaseViewModel {
 }
 
 class ScheduleEntryFilterState {
-  bool isDisplayed;
-  String entryName;
+  bool? isDisplayed;
+  String? entryName;
 
   ScheduleEntryFilterState(this.isDisplayed, this.entryName);
 }
diff --git a/lib/schedule/ui/weeklyschedule/filter/schedule_filter_page.dart b/lib/schedule/ui/weeklyschedule/filter/schedule_filter_page.dart
index 735392eb..3d5229b6 100644
--- a/lib/schedule/ui/weeklyschedule/filter/schedule_filter_page.dart
+++ b/lib/schedule/ui/weeklyschedule/filter/schedule_filter_page.dart
@@ -46,11 +46,11 @@ class ScheduleFilterPage extends StatelessWidget {
               child: PropertyChangeProvider<FilterViewModel, String>(
                 value: _viewModel,
                 child: PropertyChangeConsumer<FilterViewModel, String>(
-                    builder: (BuildContext _, FilterViewModel viewModel,
-                            Set<Object> ___) =>
+                    builder: (BuildContext _, FilterViewModel? viewModel,
+                            Set<Object>? ___) =>
                         ListView.builder(
                           shrinkWrap: true,
-                          itemCount: viewModel.filterStates.length,
+                          itemCount: viewModel!.filterStates.length,
                           itemBuilder: (context, index) =>
                               FilterStateRow(viewModel.filterStates[index]),
                         )),
@@ -74,7 +74,7 @@ class FilterStateRow extends StatefulWidget {
 }
 
 class _FilterStateRowState extends State<FilterStateRow> {
-  bool isChecked = false;
+  bool? isChecked = false;
 
   @override
   void initState() {
@@ -94,7 +94,7 @@ class _FilterStateRowState extends State<FilterStateRow> {
         });
       },
       controlAffinity: ListTileControlAffinity.leading,
-      title: Text(widget.filterState.entryName),
+      title: Text(widget.filterState.entryName!),
     );
   }
 }
diff --git a/lib/schedule/ui/weeklyschedule/schedule_entry_detail_bottom_sheet.dart b/lib/schedule/ui/weeklyschedule/schedule_entry_detail_bottom_sheet.dart
index a5ad1ea1..377906fd 100644
--- a/lib/schedule/ui/weeklyschedule/schedule_entry_detail_bottom_sheet.dart
+++ b/lib/schedule/ui/weeklyschedule/schedule_entry_detail_bottom_sheet.dart
@@ -8,19 +8,19 @@ import 'package:flutter/widgets.dart';
 import 'package:intl/intl.dart';
 
 class ScheduleEntryDetailBottomSheet extends StatelessWidget {
-  final ScheduleEntry scheduleEntry;
+  final ScheduleEntry? scheduleEntry;
 
-  const ScheduleEntryDetailBottomSheet({Key key, this.scheduleEntry})
+  const ScheduleEntryDetailBottomSheet({Key? key, this.scheduleEntry})
       : super(key: key);
 
   @override
   Widget build(BuildContext context) {
     var formatter = DateFormat.Hm(L.of(context).locale.languageCode);
-    var timeStart = formatter.format(scheduleEntry.start);
-    var timeEnd = formatter.format(scheduleEntry.end);
+    var timeStart = formatter.format(scheduleEntry!.start);
+    var timeEnd = formatter.format(scheduleEntry!.end);
 
     var typeString =
-        scheduleEntryTypeToReadableString(context, scheduleEntry.type);
+        scheduleEntryTypeToReadableString(context, scheduleEntry!.type);
 
     return Container(
       height: 400,
@@ -90,7 +90,7 @@ class ScheduleEntryDetailBottomSheet extends StatelessWidget {
                     child: Padding(
                       padding: const EdgeInsets.fromLTRB(16, 0, 0, 0),
                       child: Text(
-                        scheduleEntry.title ?? "",
+                        scheduleEntry!.title,
                         softWrap: true,
                         style: textStyleScheduleEntryBottomPageTitle(context),
                       ),
@@ -106,7 +106,7 @@ class ScheduleEntryDetailBottomSheet extends StatelessWidget {
                 children: <Widget>[
                   Expanded(
                     child: Text(
-                      scheduleEntry.professor ?? "",
+                      scheduleEntry!.professor,
                     ),
                   ),
                   Text(
@@ -116,13 +116,13 @@ class ScheduleEntryDetailBottomSheet extends StatelessWidget {
                 ],
               ),
             ),
-            scheduleEntry.room?.isEmpty ?? true
+            scheduleEntry!.room.isEmpty
                 ? Container()
                 : Padding(
                     padding: const EdgeInsets.fromLTRB(0, 8, 0, 0),
-                    child: Text(scheduleEntry.room.replaceAll(",", "\n")),
+                    child: Text(scheduleEntry!.room.replaceAll(",", "\n")),
                   ),
-            scheduleEntry.details?.isEmpty ?? true
+            scheduleEntry!.details.isEmpty
                 ? Container()
                 : Padding(
                     padding: const EdgeInsets.fromLTRB(0, 16, 0, 16),
@@ -131,9 +131,9 @@ class ScheduleEntryDetailBottomSheet extends StatelessWidget {
                       height: 1,
                     ),
                   ),
-            scheduleEntry.details?.isEmpty ?? true
+            scheduleEntry!.details.isEmpty
                 ? Container()
-                : Text(scheduleEntry.details),
+                : Text(scheduleEntry!.details),
           ],
         ),
       ),
diff --git a/lib/schedule/ui/weeklyschedule/weekly_schedule_page.dart b/lib/schedule/ui/weeklyschedule/weekly_schedule_page.dart
index baa28690..4e7f77e5 100644
--- a/lib/schedule/ui/weeklyschedule/weekly_schedule_page.dart
+++ b/lib/schedule/ui/weeklyschedule/weekly_schedule_page.dart
@@ -13,13 +13,15 @@ import 'package:provider/provider.dart';
 import 'package:url_launcher/url_launcher.dart';
 
 class WeeklySchedulePage extends StatefulWidget {
+  const WeeklySchedulePage({Key? key}) : super(key: key);
+
   @override
   _WeeklySchedulePageState createState() => _WeeklySchedulePageState();
 }
 
 class _WeeklySchedulePageState extends State<WeeklySchedulePage> {
-  WeeklyScheduleViewModel viewModel;
-  Schedule schedule;
+  late WeeklyScheduleViewModel viewModel;
+  Schedule? schedule;
 
   _WeeklySchedulePageState();
 
@@ -29,7 +31,7 @@ class _WeeklySchedulePageState extends State<WeeklySchedulePage> {
   }
 
   void _showQueryFailedSnackBar() {
-    Scaffold.of(context).showSnackBar(
+    ScaffoldMessenger.of(context).showSnackBar(
       SnackBar(
         content: Column(
           crossAxisAlignment: CrossAxisAlignment.end,
@@ -40,7 +42,7 @@ class _WeeklySchedulePageState extends State<WeeklySchedulePage> {
               child: Text(
                   L.of(context).scheduleQueryFailedOpenInBrowser.toUpperCase()),
               onPressed: () {
-                launchUrl(Uri.parse(viewModel.scheduleUrl));
+                launchUrl(Uri.parse(viewModel.scheduleUrl!));
               },
             )
           ],
@@ -51,15 +53,15 @@ class _WeeklySchedulePageState extends State<WeeklySchedulePage> {
   }
 
   void _previousWeek() async {
-    await viewModel?.previousWeek();
+    await viewModel.previousWeek();
   }
 
   void _nextWeek() async {
-    await viewModel?.nextWeek();
+    await viewModel.nextWeek();
   }
 
   void _goToToday() async {
-    await viewModel?.goToToday();
+    await viewModel.goToToday();
   }
 
   void _onScheduleEntryTap(BuildContext context, ScheduleEntry entry) {
@@ -77,7 +79,7 @@ class _WeeklySchedulePageState extends State<WeeklySchedulePage> {
 
   @override
   Widget build(BuildContext context) {
-    viewModel = Provider.of<BaseViewModel>(context);
+    viewModel = Provider.of<WeeklyScheduleViewModel>(context);
     viewModel.ensureUpdateNowTimerRunning();
 
     viewModel.setQueryFailedCallback(_showQueryFailedSnackBar);
@@ -109,9 +111,9 @@ class _WeeklySchedulePageState extends State<WeeklySchedulePage> {
                         child: PropertyChangeConsumer(
                           properties: const ["weekSchedule", "now"],
                           builder: (BuildContext context,
-                              WeeklyScheduleViewModel model, Set properties) {
+                              WeeklyScheduleViewModel? model, Set? _) {
                             return PageTransitionSwitcher(
-                              reverse: !model.didUpdateScheduleIntoFuture,
+                              reverse: !model!.didUpdateScheduleIntoFuture,
                               duration: const Duration(milliseconds: 300),
                               transitionBuilder: (Widget child,
                                       Animation<double> animation,
@@ -125,7 +127,7 @@ class _WeeklySchedulePageState extends State<WeeklySchedulePage> {
                               ),
                               child: ScheduleWidget(
                                 key: ValueKey(
-                                  model.currentDateStart.toIso8601String(),
+                                  model.currentDateStart!.toIso8601String(),
                                 ),
                                 schedule: model.weekSchedule,
                                 displayStart: model.clippedDateStart ??
@@ -146,8 +148,8 @@ class _WeeklySchedulePageState extends State<WeeklySchedulePage> {
                       PropertyChangeConsumer(
                         properties: const ["isUpdating"],
                         builder: (BuildContext context,
-                            WeeklyScheduleViewModel model, Set properties) {
-                          return model.isUpdating
+                            WeeklyScheduleViewModel? model, Set? _) {
+                          return model!.isUpdating
                               ? const LinearProgressIndicator()
                               : Container();
                         },
@@ -189,10 +191,9 @@ class _WeeklySchedulePageState extends State<WeeklySchedulePage> {
       properties: const [
         "updateFailed",
       ],
-      builder: (BuildContext context, WeeklyScheduleViewModel model,
-              Set properties) =>
+      builder: (BuildContext context, WeeklyScheduleViewModel? model, Set? _) =>
           ErrorDisplay(
-        show: model.updateFailed,
+        show: model!.updateFailed,
       ),
     );
   }
diff --git a/lib/schedule/ui/weeklyschedule/widgets/schedule_entry_alignment.dart b/lib/schedule/ui/weeklyschedule/widgets/schedule_entry_alignment.dart
index f1748038..4a753c55 100644
--- a/lib/schedule/ui/weeklyschedule/widgets/schedule_entry_alignment.dart
+++ b/lib/schedule/ui/weeklyschedule/widgets/schedule_entry_alignment.dart
@@ -3,8 +3,8 @@ import 'package:dhbwstudentapp/schedule/model/schedule_entry.dart';
 class ScheduleEntryAlignmentInformation {
   final ScheduleEntry entry;
 
-  double leftColumn;
-  double rightColumn;
+  late double leftColumn;
+  late double rightColumn;
 
   ScheduleEntryAlignmentInformation(this.entry);
 }
@@ -34,7 +34,7 @@ class ScheduleEntryAlignmentAlgorithm {
 
     List<List<ScheduleEntryAlignmentInformation>> columns = [[]];
 
-    DateTime lastEventEnd;
+    DateTime? lastEventEnd;
 
     for (var event in events) {
       if (!event.entry.start
diff --git a/lib/schedule/ui/weeklyschedule/widgets/schedule_entry_widget.dart b/lib/schedule/ui/weeklyschedule/widgets/schedule_entry_widget.dart
index 8511ad5c..db798f14 100644
--- a/lib/schedule/ui/weeklyschedule/widgets/schedule_entry_widget.dart
+++ b/lib/schedule/ui/weeklyschedule/widgets/schedule_entry_widget.dart
@@ -7,11 +7,11 @@ typedef ScheduleEntryTapCallback = Function(ScheduleEntry entry);
 
 class ScheduleEntryWidget extends StatelessWidget {
   final ScheduleEntry scheduleEntry;
-  final ScheduleEntryTapCallback onScheduleEntryTap;
+  final ScheduleEntryTapCallback? onScheduleEntryTap;
 
   const ScheduleEntryWidget({
-    Key key,
-    this.scheduleEntry,
+    Key? key,
+    required this.scheduleEntry,
     this.onScheduleEntryTap,
   }) : super(key: key);
 
@@ -25,7 +25,7 @@ class ScheduleEntryWidget extends StatelessWidget {
       margin: const EdgeInsets.fromLTRB(1, 0, 1, 0),
       child: InkWell(
         onTap: () {
-          if (onScheduleEntryTap != null) onScheduleEntryTap(scheduleEntry);
+          if (onScheduleEntryTap != null) onScheduleEntryTap!(scheduleEntry);
         },
         child: Padding(
           padding: const EdgeInsets.fromLTRB(4, 4, 4, 4),
diff --git a/lib/schedule/ui/weeklyschedule/widgets/schedule_grid.dart b/lib/schedule/ui/weeklyschedule/widgets/schedule_grid.dart
index c306466c..361e9249 100644
--- a/lib/schedule/ui/weeklyschedule/widgets/schedule_grid.dart
+++ b/lib/schedule/ui/weeklyschedule/widgets/schedule_grid.dart
@@ -2,8 +2,8 @@ import 'package:flutter/material.dart';
 import 'package:flutter/widgets.dart';
 
 class ScheduleGrid extends CustomPaint {
-  final int fromHour;
-  final int toHour;
+  final int? fromHour;
+  final int? toHour;
   final double timeLabelsWidth;
   final double dateLabelsHeight;
   final int columns;
@@ -23,8 +23,8 @@ class ScheduleGrid extends CustomPaint {
 }
 
 class ScheduleGridCustomPaint extends CustomPainter {
-  final int fromHour;
-  final int toHour;
+  final int? fromHour;
+  final int? toHour;
   final double timeLabelsWidth;
   final double dateLabelsHeight;
   final int columns;
@@ -45,7 +45,7 @@ class ScheduleGridCustomPaint extends CustomPainter {
       ..color = gridLineColor
       ..strokeWidth = 1;
 
-    var lines = toHour - fromHour;
+    var lines = toHour! - fromHour!;
 
     drawHorizontalLines(lines, size, canvas, secondaryPaint);
     drawVerticalLines(size, canvas, secondaryPaint);
diff --git a/lib/schedule/ui/weeklyschedule/widgets/schedule_past_overlay.dart b/lib/schedule/ui/weeklyschedule/widgets/schedule_past_overlay.dart
index 94234607..ae98def9 100644
--- a/lib/schedule/ui/weeklyschedule/widgets/schedule_past_overlay.dart
+++ b/lib/schedule/ui/weeklyschedule/widgets/schedule_past_overlay.dart
@@ -5,11 +5,11 @@ import 'package:flutter/material.dart';
 import 'package:flutter/widgets.dart';
 
 class SchedulePastOverlay extends CustomPaint {
-  final DateTime fromDate;
-  final DateTime toDate;
-  final DateTime now;
-  final int fromHour;
-  final int toHour;
+  final DateTime? fromDate;
+  final DateTime? toDate;
+  final DateTime? now;
+  final int? fromHour;
+  final int? toHour;
   final int columns;
   final Color overlayColor;
 
@@ -35,11 +35,11 @@ class SchedulePastOverlay extends CustomPaint {
 }
 
 class SchedulePastOverlayCustomPaint extends CustomPainter {
-  final DateTime fromDate;
-  final DateTime toDate;
-  final DateTime now;
-  final int fromHour;
-  final int toHour;
+  final DateTime? fromDate;
+  final DateTime? toDate;
+  final DateTime? now;
+  final int? fromHour;
+  final int? toHour;
   final int columns;
   final Color overlayColor;
 
@@ -59,9 +59,9 @@ class SchedulePastOverlayCustomPaint extends CustomPainter {
       ..color = overlayColor
       ..strokeWidth = 1;
 
-    if (now.isBefore(toDate) && now.isAfter(fromDate)) {
+    if (now!.isBefore(toDate!) && now!.isAfter(fromDate!)) {
       drawPartialPastOverlay(canvas, size, overlayPaint);
-    } else if (now.isAfter(toDate)) {
+    } else if (now!.isAfter(toDate!)) {
       drawCompletePastOverlay(canvas, size, overlayPaint);
     }
   }
@@ -78,7 +78,7 @@ class SchedulePastOverlayCustomPaint extends CustomPainter {
   }
 
   void drawPartialPastOverlay(Canvas canvas, Size size, Paint overlayPaint) {
-    var difference = now.difference(fromDate);
+    var difference = now!.difference(fromDate!);
     var differenceInDays = (difference.inHours / 24).floor();
     var dayWidth = size.width / columns;
 
@@ -93,9 +93,9 @@ class SchedulePastOverlayCustomPaint extends CustomPainter {
     );
 
     var leftoverMinutes =
-        difference.inMinutes - (differenceInDays * 24 * 60) - (60 * fromHour);
+        difference.inMinutes - (differenceInDays * 24 * 60) - (60 * fromHour!);
 
-    var displayedMinutes = (toHour - fromHour) * 60;
+    var displayedMinutes = (toHour! - fromHour!) * 60;
     var leftoverHeight = leftoverMinutes * (size.height / displayedMinutes);
 
     if (leftoverHeight > 0) {
diff --git a/lib/schedule/ui/weeklyschedule/widgets/schedule_widget.dart b/lib/schedule/ui/weeklyschedule/widgets/schedule_widget.dart
index 59fcaeef..748b3290 100644
--- a/lib/schedule/ui/weeklyschedule/widgets/schedule_widget.dart
+++ b/lib/schedule/ui/weeklyschedule/widgets/schedule_widget.dart
@@ -13,16 +13,16 @@ import 'package:flutter/widgets.dart';
 import 'package:intl/intl.dart';
 
 class ScheduleWidget extends StatelessWidget {
-  final Schedule schedule;
-  final DateTime displayStart;
-  final DateTime displayEnd;
-  final DateTime now;
-  final int displayStartHour;
-  final int displayEndHour;
-  final ScheduleEntryTapCallback onScheduleEntryTap;
+  final Schedule? schedule;
+  final DateTime? displayStart;
+  final DateTime? displayEnd;
+  final DateTime? now;
+  final int? displayStartHour;
+  final int? displayEndHour;
+  final ScheduleEntryTapCallback? onScheduleEntryTap;
 
   const ScheduleWidget({
-    Key key,
+    Key? key,
     this.schedule,
     this.displayStart,
     this.displayEnd,
@@ -48,7 +48,7 @@ class ScheduleWidget extends StatelessWidget {
     var timeLabelsWidth = 50.0;
 
     var hourHeight =
-        (height - dayLabelsHeight) / (displayEndHour - displayStartHour);
+        (height - dayLabelsHeight) / (displayEndHour! - displayStartHour!);
     var minuteHeight = hourHeight / 60;
 
     var days = calculateDisplayedDays();
@@ -112,7 +112,7 @@ class ScheduleWidget extends StatelessWidget {
 
   int calculateDisplayedDays() {
     var startEndDifference =
-        toStartOfDay(displayEnd)?.difference(toStartOfDay(displayStart));
+        toStartOfDay(displayEnd)?.difference(toStartOfDay(displayStart)!);
 
     var days = (startEndDifference?.inDays ?? 0) + 1;
 
@@ -135,12 +135,12 @@ class ScheduleWidget extends StatelessWidget {
   ) {
     var labelWidgets = <Widget>[];
 
-    for (var i = displayStartHour; i < displayEndHour; i++) {
+    for (var i = displayStartHour!; i < displayEndHour!; i++) {
       var hourLabelText = i.toString() + ":00";
 
       labelWidgets.add(
         Positioned(
-          top: rowHeight * (i - displayStartHour) + dayLabelHeight,
+          top: rowHeight * (i - displayStartHour!) + dayLabelHeight,
           left: 0,
           child: Padding(
             padding: const EdgeInsets.fromLTRB(4, 4, 4, 8),
@@ -155,11 +155,11 @@ class ScheduleWidget extends StatelessWidget {
     var dayFormatter = DateFormat("E", L.of(context).locale.languageCode);
     var dateFormatter = DateFormat("d. MMM", L.of(context).locale.languageCode);
 
-    var loopEnd = toStartOfDay(tomorrow(displayEnd));
+    var loopEnd = toStartOfDay(tomorrow(displayEnd))!;
 
-    for (var columnDate = toStartOfDay(displayStart);
+    for (var columnDate = toStartOfDay(displayStart)!;
         columnDate.isBefore(loopEnd);
-        columnDate = tomorrow(columnDate)) {
+        columnDate = tomorrow(columnDate)!) {
       labelWidgets.add(
         Positioned(
           top: 0,
@@ -192,20 +192,20 @@ class ScheduleWidget extends StatelessWidget {
   List<Widget> buildEntryWidgets(
       double hourHeight, double minuteHeight, double width, int columns) {
     if (schedule == null) return <Widget>[];
-    if (schedule.entries.isEmpty) return <Widget>[];
+    if (schedule!.entries.isEmpty) return <Widget>[];
 
     var entryWidgets = <Widget>[];
 
     var columnWidth = width / columns;
 
-    DateTime columnStartDate = toStartOfDay(displayStart);
-    DateTime columnEndDate = tomorrow(columnStartDate);
+    DateTime? columnStartDate = toStartOfDay(displayStart);
+    DateTime? columnEndDate = tomorrow(columnStartDate);
 
     for (int i = 0; i < columns; i++) {
       var xPosition = columnWidth * i;
       var maxWidth = columnWidth;
 
-      var columnSchedule = schedule.trim(columnStartDate, columnEndDate);
+      var columnSchedule = schedule!.trim(columnStartDate, columnEndDate);
 
       entryWidgets.addAll(buildEntryWidgetsForColumn(
         maxWidth,
@@ -237,10 +237,10 @@ class ScheduleWidget extends StatelessWidget {
     for (var value in laidOutEntries) {
       var entry = value.entry;
 
-      var yStart = hourHeight * (entry.start.hour - displayStartHour) +
+      var yStart = hourHeight * (entry.start.hour - displayStartHour!) +
           minuteHeight * entry.start.minute;
 
-      var yEnd = hourHeight * (entry.end.hour - displayStartHour) +
+      var yEnd = hourHeight * (entry.end.hour - displayStartHour!) +
           minuteHeight * entry.end.minute;
 
       var entryLeft = maxWidth * value.leftColumn;
diff --git a/lib/schedule/ui/widgets/enter_dualis_credentials_dialog.dart b/lib/schedule/ui/widgets/enter_dualis_credentials_dialog.dart
index cca38e58..eec5214e 100644
--- a/lib/schedule/ui/widgets/enter_dualis_credentials_dialog.dart
+++ b/lib/schedule/ui/widgets/enter_dualis_credentials_dialog.dart
@@ -9,13 +9,9 @@ class EnterDualisCredentialsDialog {
   final PreferencesProvider _preferencesProvider;
   final ScheduleSourceProvider _scheduleSourceProvider;
 
-  final TextEditingController _usernameEditingController =
-      TextEditingController();
-  final TextEditingController _passwordEditingController =
-      TextEditingController();
+  final _controller = CredentialsEditingController();
 
-  String username;
-  String password;
+  Credentials? _credentials;
 
   EnterDualisCredentialsDialog(
     this._preferencesProvider,
@@ -23,9 +19,7 @@ class EnterDualisCredentialsDialog {
   );
 
   Future show(BuildContext context) async {
-    var credentials = await _preferencesProvider.loadDualisCredentials();
-    username = credentials.username;
-    password = credentials.password;
+    _credentials = await _preferencesProvider.loadDualisCredentials();
 
     await showDialog(
       context: context,
@@ -34,11 +28,9 @@ class EnterDualisCredentialsDialog {
   }
 
   AlertDialog _buildDialog(BuildContext context) {
-    if (_usernameEditingController.text != username) {
-      _usernameEditingController.text = username;
-    }
-    if (_passwordEditingController.text != password) {
-      _passwordEditingController.text = password;
+    final credentials = _credentials;
+    if (credentials != null) {
+      _controller.credentials = credentials;
     }
 
     return AlertDialog(
@@ -55,8 +47,7 @@ class EnterDualisCredentialsDialog {
             ),
           ),
           LoginCredentialsWidget(
-            usernameEditingController: _usernameEditingController,
-            passwordEditingController: _passwordEditingController,
+            controller: _controller,
             onSubmitted: () async {},
           ),
         ],
@@ -70,11 +61,9 @@ class EnterDualisCredentialsDialog {
       TextButton(
         child: Text(L.of(context).dialogOk.toUpperCase()),
         onPressed: () async {
+          // TODO: [Leptopoda] validate credentials like [DualisLoginViewModel]
           await _preferencesProvider.storeDualisCredentials(
-            Credentials(
-              _usernameEditingController.text,
-              _passwordEditingController.text,
-            ),
+            _controller.credentials,
           );
           await _scheduleSourceProvider.setupForDualis();
 
diff --git a/lib/schedule/ui/widgets/enter_ical_url.dart b/lib/schedule/ui/widgets/enter_ical_url.dart
index 25cc1d17..b563d723 100644
--- a/lib/schedule/ui/widgets/enter_ical_url.dart
+++ b/lib/schedule/ui/widgets/enter_ical_url.dart
@@ -12,18 +12,18 @@ class EnterIcalDialog extends EnterUrlDialog {
   EnterIcalDialog(this._preferencesProvider, this._scheduleSource);
 
   @override
-  Future<String> loadUrl() {
+  Future<String?> loadUrl() {
     return _preferencesProvider.getIcalUrl();
   }
 
   @override
-  Future saveUrl(String url) async {
+  Future saveUrl(String? url) async {
     await _scheduleSource.setupForIcal(url);
   }
 
   @override
-  bool isValidUrl(String url) {
-    return IcalScheduleSource.isValidUrl(url);
+  bool isValidUrl(String? url) {
+    return IcalScheduleSource.isValidUrl(url!);
   }
 
   @override
diff --git a/lib/schedule/ui/widgets/enter_rapla_url_dialog.dart b/lib/schedule/ui/widgets/enter_rapla_url_dialog.dart
index 0bc5167b..c2ed49b2 100644
--- a/lib/schedule/ui/widgets/enter_rapla_url_dialog.dart
+++ b/lib/schedule/ui/widgets/enter_rapla_url_dialog.dart
@@ -15,7 +15,7 @@ class EnterRaplaUrlDialog extends EnterUrlDialog {
   EnterRaplaUrlDialog(this._preferencesProvider, this._scheduleSource);
 
   @override
-  Future saveUrl(String url) async {
+  Future saveUrl(String? url) async {
     await _scheduleSource.setupForRapla(url);
   }
 
@@ -25,8 +25,8 @@ class EnterRaplaUrlDialog extends EnterUrlDialog {
   }
 
   @override
-  bool isValidUrl(String url) {
-    return RaplaScheduleSource.isValidUrl(url);
+  bool isValidUrl(String? url) {
+    return RaplaScheduleSource.isValidUrl(url!);
   }
 
   @override
diff --git a/lib/schedule/ui/widgets/enter_url_dialog.dart b/lib/schedule/ui/widgets/enter_url_dialog.dart
index 58a65dc4..fc09625a 100644
--- a/lib/schedule/ui/widgets/enter_url_dialog.dart
+++ b/lib/schedule/ui/widgets/enter_url_dialog.dart
@@ -49,7 +49,7 @@ abstract class EnterUrlDialog {
                     builder: (
                       BuildContext context,
                       ValueNotifier<String> url,
-                      Widget child,
+                      Widget? child,
                     ) {
                       if (_urlTextController.text != url.value)
                         _urlTextController.text = url.value;
@@ -58,7 +58,7 @@ abstract class EnterUrlDialog {
                         builder: (
                           BuildContext context,
                           ValueNotifier<bool> hasUrlError,
-                          Widget child,
+                          Widget? child,
                         ) =>
                             TextField(
                           controller: _urlTextController,
@@ -103,7 +103,7 @@ abstract class EnterUrlDialog {
         value: _hasUrlError,
         child: Consumer(
           builder: (BuildContext context, ValueNotifier<bool> hasUrlError,
-                  Widget child) =>
+                  Widget? child) =>
               TextButton(
             child: Text(L.of(context).dialogOk.toUpperCase()),
             onPressed: hasUrlError.value
@@ -120,15 +120,15 @@ abstract class EnterUrlDialog {
   }
 
   Future _pasteUrl() async {
-    ClipboardData data = await Clipboard.getData('text/plain');
+    ClipboardData? data = await Clipboard.getData('text/plain');
 
     if (data?.text != null) {
-      _setUrl(data.text);
+      _setUrl(data!.text!);
     }
   }
 
   Future _loadUrl() async {
-    _url.value = await loadUrl();
+    _url.value = await loadUrl() ?? "";
   }
 
   void _setUrl(String url) {
@@ -140,9 +140,9 @@ abstract class EnterUrlDialog {
     _hasUrlError.value = !isValidUrl(_url.value);
   }
 
-  bool isValidUrl(String url);
-  Future saveUrl(String url);
-  Future<String> loadUrl();
+  bool isValidUrl(String? url);
+  Future saveUrl(String? url);
+  Future<String?> loadUrl();
 
   String title(BuildContext context);
   String message(BuildContext context);
diff --git a/lib/schedule/ui/widgets/schedule_empty_state.dart b/lib/schedule/ui/widgets/schedule_empty_state.dart
index c52a1e59..37c846cc 100644
--- a/lib/schedule/ui/widgets/schedule_empty_state.dart
+++ b/lib/schedule/ui/widgets/schedule_empty_state.dart
@@ -32,7 +32,7 @@ class ScheduleEmptyState extends StatelessWidget {
           child: ClipRRect(
             child: Padding(
               padding: const EdgeInsets.all(32),
-              child: Image.asset(image[Theme.of(context).brightness]),
+              child: Image.asset(image[Theme.of(context).brightness]!),
             ),
           ),
         )
diff --git a/lib/schedule/ui/widgets/select_mannheim_course_dialog.dart b/lib/schedule/ui/widgets/select_mannheim_course_dialog.dart
index 70943bb4..e828b847 100644
--- a/lib/schedule/ui/widgets/select_mannheim_course_dialog.dart
+++ b/lib/schedule/ui/widgets/select_mannheim_course_dialog.dart
@@ -9,7 +9,7 @@ import 'package:property_change_notifier/property_change_notifier.dart';
 class SelectMannheimCourseDialog {
   final ScheduleSourceProvider _scheduleSourceProvider;
 
-  MannheimViewModel _mannheimViewModel;
+  late MannheimViewModel _mannheimViewModel;
 
   SelectMannheimCourseDialog(
     this._scheduleSourceProvider,
diff --git a/lib/schedule/ui/widgets/select_source_dialog.dart b/lib/schedule/ui/widgets/select_source_dialog.dart
index 83d03fd7..5ec9f9cd 100644
--- a/lib/schedule/ui/widgets/select_source_dialog.dart
+++ b/lib/schedule/ui/widgets/select_source_dialog.dart
@@ -13,7 +13,7 @@ class SelectSourceDialog {
   final PreferencesProvider _preferencesProvider;
   final ScheduleSourceProvider _scheduleSourceProvider;
 
-  ScheduleSourceType _currentScheduleSource;
+  ScheduleSourceType? _currentScheduleSource;
 
   SelectSourceDialog(this._preferencesProvider, this._scheduleSourceProvider);
 
@@ -41,31 +41,31 @@ class SelectSourceDialog {
         RadioListTile(
           groupValue: _currentScheduleSource,
           value: ScheduleSourceType.Rapla,
-          onChanged: (v) => sourceSelected(v, context),
+          onChanged: (dynamic v) => sourceSelected(v, context),
           title: Text(L.of(context).scheduleSourceTypeRapla),
         ),
         RadioListTile(
           groupValue: _currentScheduleSource,
           value: ScheduleSourceType.Dualis,
-          onChanged: (v) => sourceSelected(v, context),
+          onChanged: (dynamic v) => sourceSelected(v, context),
           title: Text(L.of(context).scheduleSourceTypeDualis),
         ),
         RadioListTile(
           groupValue: _currentScheduleSource,
           value: ScheduleSourceType.Mannheim,
-          onChanged: (v) => sourceSelected(v, context),
+          onChanged: (dynamic v) => sourceSelected(v, context),
           title: Text(L.of(context).scheduleSourceTypeMannheim),
         ),
         RadioListTile(
           groupValue: _currentScheduleSource,
           value: ScheduleSourceType.Ical,
-          onChanged: (v) => sourceSelected(v, context),
+          onChanged: (dynamic v) => sourceSelected(v, context),
           title: Text(L.of(context).scheduleSourceTypeIcal),
         ),
         RadioListTile(
           groupValue: _currentScheduleSource,
           value: ScheduleSourceType.None,
-          onChanged: (v) => sourceSelected(v, context),
+          onChanged: (dynamic v) => sourceSelected(v, context),
           title: Text(L.of(context).scheduleSourceTypeNone),
         )
       ],
@@ -76,6 +76,7 @@ class SelectSourceDialog {
     ScheduleSourceType type,
     BuildContext context,
   ) async {
+    // TODO: [Leptopoda] only switch the type when the setup is completed.
     _preferencesProvider.setScheduleSourceType(type.index);
 
     Navigator.of(context).pop();
diff --git a/lib/ui/banner_widget.dart b/lib/ui/banner_widget.dart
index 7700d6e2..f993f420 100644
--- a/lib/ui/banner_widget.dart
+++ b/lib/ui/banner_widget.dart
@@ -5,7 +5,11 @@ class BannerWidget extends StatelessWidget {
   final String buttonText;
   final VoidCallback onButtonTap;
 
-  const BannerWidget({Key key, this.message, this.buttonText, this.onButtonTap})
+  const BannerWidget(
+      {Key? key,
+      required this.message,
+      required this.buttonText,
+      required this.onButtonTap})
       : super(key: key);
 
   @override
@@ -27,23 +31,19 @@ class BannerWidget extends StatelessWidget {
           mainAxisSize: MainAxisSize.min,
           crossAxisAlignment: CrossAxisAlignment.stretch,
           children: <Widget>[
-            Text(
-              message,
+            Text(message),
+            Column(
+              crossAxisAlignment: CrossAxisAlignment.end,
+              children: <Widget>[
+                Padding(
+                  padding: const EdgeInsets.fromLTRB(0, 12, 0, 0),
+                  child: TextButton(
+                    child: Text(buttonText),
+                    onPressed: onButtonTap,
+                  ),
+                ),
+              ],
             ),
-            buttonText != null
-                ? Column(
-                    crossAxisAlignment: CrossAxisAlignment.end,
-                    children: <Widget>[
-                      Padding(
-                        padding: const EdgeInsets.fromLTRB(0, 12, 0, 0),
-                        child: TextButton(
-                          child: Text(buttonText),
-                          onPressed: onButtonTap,
-                        ),
-                      ),
-                    ],
-                  )
-                : Container(),
           ],
         ),
       ),
diff --git a/lib/ui/login_credentials_widget.dart b/lib/ui/login_credentials_widget.dart
index b8ee1134..c3e70936 100644
--- a/lib/ui/login_credentials_widget.dart
+++ b/lib/ui/login_credentials_widget.dart
@@ -1,45 +1,30 @@
 import 'package:dhbwstudentapp/common/i18n/localizations.dart';
+import 'package:dhbwstudentapp/dualis/model/credentials.dart';
 import 'package:flutter/material.dart';
 
 class LoginCredentialsWidget extends StatefulWidget {
-  final TextEditingController usernameEditingController;
-  final TextEditingController passwordEditingController;
+  final CredentialsEditingController controller;
   final Function onSubmitted;
 
   const LoginCredentialsWidget({
-    Key key,
-    this.usernameEditingController,
-    this.passwordEditingController,
-    this.onSubmitted,
+    Key? key,
+    required this.controller,
+    required this.onSubmitted,
   }) : super(key: key);
 
   @override
-  _LoginCredentialsWidgetState createState() => _LoginCredentialsWidgetState(
-        usernameEditingController,
-        passwordEditingController,
-        onSubmitted,
-      );
+  _LoginCredentialsWidgetState createState() => _LoginCredentialsWidgetState();
 }
 
 class _LoginCredentialsWidgetState extends State<LoginCredentialsWidget> {
-  final TextEditingController _usernameEditingController;
-  final TextEditingController _passwordEditingController;
-  final Function _onSubmitted;
-
   final _focus = FocusNode();
 
-  _LoginCredentialsWidgetState(
-    this._usernameEditingController,
-    this._passwordEditingController,
-    this._onSubmitted,
-  );
-
   @override
   Widget build(BuildContext context) {
     return Column(
       children: <Widget>[
         TextField(
-          controller: _usernameEditingController,
+          controller: widget.controller.username,
           decoration: InputDecoration(
             hintText: L.of(context).loginUsername,
             icon: Icon(Icons.alternate_email),
@@ -50,7 +35,7 @@ class _LoginCredentialsWidgetState extends State<LoginCredentialsWidget> {
           textInputAction: TextInputAction.next,
         ),
         TextField(
-          controller: _passwordEditingController,
+          controller: widget.controller.password,
           obscureText: true,
           decoration: InputDecoration(
             hintText: L.of(context).loginPassword,
@@ -58,9 +43,7 @@ class _LoginCredentialsWidgetState extends State<LoginCredentialsWidget> {
           ),
           focusNode: _focus,
           onSubmitted: (v) {
-            if (_onSubmitted != null) {
-              _onSubmitted();
-            }
+            widget.onSubmitted();
           },
         ),
       ],
diff --git a/lib/ui/main_page.dart b/lib/ui/main_page.dart
index 9584c87c..47cfa706 100644
--- a/lib/ui/main_page.dart
+++ b/lib/ui/main_page.dart
@@ -15,6 +15,8 @@ import 'package:provider/provider.dart';
 /// To navigate to a new route inside this widget use the [NavigatorKey.mainKey]
 ///
 class MainPage extends StatefulWidget {
+  const MainPage({Key? key}) : super(key: key);
+
   @override
   _MainPageState createState() => _MainPageState();
 }
@@ -46,7 +48,7 @@ class _MainPageState extends State<MainPage> with NavigatorObserver {
     return ChangeNotifierProvider.value(
       value: _currentEntryIndex,
       child: Consumer<ValueNotifier<int>>(
-        builder: (BuildContext context, value, Widget child) {
+        builder: (BuildContext context, value, Widget? child) {
           Widget content;
 
           if (PlatformUtil.isPhone() || PlatformUtil.isPortrait(context)) {
@@ -64,11 +66,11 @@ class _MainPageState extends State<MainPage> with NavigatorObserver {
   Widget buildPhoneLayout(BuildContext context, Navigator navigator) {
     return WillPopScope(
       onWillPop: () async {
-        var canPop = NavigatorKey.mainKey.currentState.canPop();
+        var canPop = NavigatorKey.mainKey.currentState!.canPop();
 
         if (!canPop) return true;
 
-        NavigatorKey.mainKey.currentState.pop();
+        NavigatorKey.mainKey.currentState!.pop();
 
         return false;
       },
@@ -135,7 +137,7 @@ class _MainPageState extends State<MainPage> with NavigatorObserver {
 
     for (var entry in navigationEntries) {
       drawerEntries.add(DrawerNavigationEntry(
-        entry.icon(context),
+        entry.icon,
         entry.title(context),
       ));
     }
@@ -146,7 +148,7 @@ class _MainPageState extends State<MainPage> with NavigatorObserver {
   void _onNavigationTapped(int index) {
     _currentEntryIndex.value = index;
 
-    NavigatorKey.mainKey.currentState
+    NavigatorKey.mainKey.currentState!
         .pushNamedAndRemoveUntil(currentEntry.route, (route) {
       return route.settings.name == navigationEntries[0].route;
     });
@@ -160,7 +162,7 @@ class _MainPageState extends State<MainPage> with NavigatorObserver {
     }
   }
 
-  void updateNavigationDrawer(String routeName) {
+  void updateNavigationDrawer(String? routeName) {
     for (int i = 0; i < navigationEntries.length; i++) {
       if (navigationEntries[i].route == routeName) {
         _currentEntryIndex.value = i;
@@ -170,17 +172,17 @@ class _MainPageState extends State<MainPage> with NavigatorObserver {
   }
 
   @override
-  void didPop(Route<dynamic> route, Route<dynamic> previousRoute) {
-    updateNavigationDrawer(previousRoute.settings.name);
+  void didPop(Route<dynamic> route, Route<dynamic>? previousRoute) {
+    updateNavigationDrawer(previousRoute!.settings.name);
   }
 
   @override
-  void didPush(Route<dynamic> route, Route<dynamic> previousRoute) {
+  void didPush(Route<dynamic> route, Route<dynamic>? previousRoute) {
     updateNavigationDrawer(route.settings.name);
   }
 
   @override
-  void didReplace({Route<dynamic> newRoute, Route<dynamic> oldRoute}) {
-    updateNavigationDrawer(newRoute.settings.name);
+  void didReplace({Route<dynamic>? newRoute, Route<dynamic>? oldRoute}) {
+    updateNavigationDrawer(newRoute!.settings.name);
   }
 }
diff --git a/lib/ui/navigation/navigation_entry.dart b/lib/ui/navigation/navigation_entry.dart
index dedbf575..4c123c22 100644
--- a/lib/ui/navigation/navigation_entry.dart
+++ b/lib/ui/navigation/navigation_entry.dart
@@ -2,38 +2,30 @@ import 'package:dhbwstudentapp/common/ui/viewmodels/base_view_model.dart';
 import 'package:flutter/material.dart';
 import 'package:provider/provider.dart';
 
-abstract class NavigationEntry {
-  BaseViewModel _viewModel;
+abstract class NavigationEntry<T extends BaseViewModel> {
+  T? _viewModel;
 
   String get route;
 
   String title(BuildContext context);
 
-  Widget icon(BuildContext context);
+  Icon get icon;
 
   Widget buildRoute(BuildContext context) {
-    var model = viewModel();
-    if (model != null) {
-      return ChangeNotifierProvider.value(
-        value: model,
-        child: build(context),
-      );
-    } else {
-      return build(context);
-    }
+    return ChangeNotifierProvider.value(
+      value: model,
+      child: build(context),
+    );
   }
 
   Widget build(BuildContext context);
 
-  BaseViewModel viewModel() {
-    if (_viewModel == null) {
-      _viewModel = initViewModel();
-    }
-
-    return _viewModel;
+  T get model {
+    return _viewModel ??= initViewModel();
   }
 
-  BaseViewModel initViewModel() => null;
+  T initViewModel();
 
-  List<Widget> appBarActions(BuildContext context) => [];
+  // TODO: [Leptopoda] clean up but not null related so something for later
+  List<Widget> appBarActions(BuildContext context);
 }
diff --git a/lib/ui/navigation/router.dart b/lib/ui/navigation/router.dart
index 72c5fe58..f17e4cd7 100644
--- a/lib/ui/navigation/router.dart
+++ b/lib/ui/navigation/router.dart
@@ -18,7 +18,7 @@ final List<NavigationEntry> navigationEntries = [
 Route<dynamic> generateDrawerRoute(RouteSettings settings) {
   print("=== === === === === === Navigating to: ${settings.name}");
 
-  WidgetBuilder widget;
+  WidgetBuilder? widget;
 
   for (var route in navigationEntries) {
     if (route.route == settings.name) {
@@ -28,14 +28,14 @@ Route<dynamic> generateDrawerRoute(RouteSettings settings) {
   }
 
   if (widget == null) {
-    print("Failed to navigate to: " + settings.name);
+    print("Failed to navigate to: " + settings.name!);
     widget = (BuildContext context) => Container();
   }
 
   return PageRouteBuilder(
     settings: settings,
     transitionDuration: const Duration(milliseconds: 200),
-    pageBuilder: (context, animation, secondaryAnimation) => widget(context),
+    pageBuilder: (context, animation, secondaryAnimation) => widget!(context),
     transitionsBuilder: (context, animation, secondaryAnimation, child) {
       const offsetBegin = Offset(0.0, 0.005);
       final offsetEnd = Offset.zero;
@@ -76,7 +76,7 @@ Route<dynamic> generateRoute(RouteSettings settings) {
       target = SettingsPage();
       break;
     default:
-      print("Failed to navigate to: " + settings.name);
+      print("Failed to navigate to: " + settings.name!);
       target = Container();
   }
 
diff --git a/lib/ui/navigation_drawer.dart b/lib/ui/navigation_drawer.dart
index 6266b583..abc21d8a 100644
--- a/lib/ui/navigation_drawer.dart
+++ b/lib/ui/navigation_drawer.dart
@@ -16,10 +16,10 @@ class NavigationDrawer extends StatelessWidget {
   final bool isInDrawer;
 
   const NavigationDrawer({
-    Key key,
-    this.selectedIndex,
-    this.onTap,
-    this.entries,
+    Key? key,
+    required this.selectedIndex,
+    required this.onTap,
+    required this.entries,
     this.isInDrawer = true,
   }) : super(key: key);
 
@@ -81,10 +81,10 @@ class NavigationDrawer extends StatelessWidget {
 
   Widget _createDrawerItem(
     BuildContext context, {
-    Widget icon,
-    String text,
-    bool isSelected,
-    int index,
+    required Widget icon,
+    required String text,
+    required bool isSelected,
+    required int index,
   }) {
     return Padding(
       padding: const EdgeInsets.fromLTRB(8, 8, 8, 8),
@@ -169,7 +169,7 @@ class NavigationDrawer extends StatelessWidget {
 }
 
 class DrawerNavigationEntry {
-  final Widget icon;
+  final Icon icon;
   final String title;
 
   DrawerNavigationEntry(this.icon, this.title);
diff --git a/lib/ui/onboarding/onboardin_step.dart b/lib/ui/onboarding/onboardin_step.dart
index 37ee31aa..c800089f 100644
--- a/lib/ui/onboarding/onboardin_step.dart
+++ b/lib/ui/onboarding/onboardin_step.dart
@@ -17,7 +17,7 @@ abstract class OnboardingStep {
 
   OnboardingStepViewModel viewModel();
 
-  String nextStep();
+  String? nextStep();
 }
 
 class SelectSourceOnboardingStep extends OnboardingStep {
@@ -31,7 +31,7 @@ class SelectSourceOnboardingStep extends OnboardingStep {
   }
 
   @override
-  String nextStep() {
+  String? nextStep() {
     return _viewModel.nextStep();
   }
 
@@ -53,7 +53,7 @@ class DualisCredentialsOnboardingStep extends OnboardingStep {
   }
 
   @override
-  String nextStep() {
+  String? nextStep() {
     return null;
   }
 
@@ -108,7 +108,7 @@ class IcalOnboardingStep extends OnboardingStep {
 }
 
 class MannheimOnboardingStep extends OnboardingStep {
-  MannheimViewModel _viewModel;
+  MannheimViewModel? _viewModel;
 
   @override
   Widget buildContent(BuildContext context) {
@@ -122,12 +122,8 @@ class MannheimOnboardingStep extends OnboardingStep {
 
   @override
   OnboardingStepViewModel viewModel() {
-    if (_viewModel == null) {
-      _viewModel = MannheimViewModel(
-        KiwiContainer().resolve(),
-      );
-    }
-
-    return _viewModel;
+    return _viewModel ??= MannheimViewModel(
+      KiwiContainer().resolve(),
+    );
   }
 }
diff --git a/lib/ui/onboarding/onboarding_page.dart b/lib/ui/onboarding/onboarding_page.dart
index 58a9f086..651f2bbe 100644
--- a/lib/ui/onboarding/onboarding_page.dart
+++ b/lib/ui/onboarding/onboarding_page.dart
@@ -1,5 +1,4 @@
 import 'package:animations/animations.dart';
-import 'package:dhbwstudentapp/common/ui/viewmodels/base_view_model.dart';
 import 'package:dhbwstudentapp/common/ui/viewmodels/root_view_model.dart';
 import 'package:dhbwstudentapp/ui/onboarding/viewmodels/onboarding_view_model.dart';
 import 'package:dhbwstudentapp/ui/onboarding/viewmodels/onboarding_view_model_base.dart';
@@ -10,7 +9,7 @@ import 'package:kiwi/kiwi.dart';
 import 'package:property_change_notifier/property_change_notifier.dart';
 
 class OnboardingPage extends StatefulWidget {
-  const OnboardingPage({Key key}) : super(key: key);
+  const OnboardingPage({Key? key}) : super(key: key);
 
   @override
   _OnboardingPageState createState() => _OnboardingPageState();
@@ -18,8 +17,8 @@ class OnboardingPage extends StatefulWidget {
 
 class _OnboardingPageState extends State<OnboardingPage>
     with TickerProviderStateMixin {
-  AnimationController _controller;
-  OnboardingViewModel viewModel;
+  late AnimationController _controller;
+  late OnboardingViewModel viewModel;
 
   _OnboardingPageState();
 
@@ -35,7 +34,7 @@ class _OnboardingPageState extends State<OnboardingPage>
     viewModel.addListener(
       () async {
         await _controller.animateTo(
-            viewModel.stepIndex / viewModel.onboardingSteps,
+            viewModel.stepIndex! / viewModel.onboardingSteps,
             curve: Curves.ease,
             duration: const Duration(milliseconds: 300));
       },
@@ -75,12 +74,12 @@ class _OnboardingPageState extends State<OnboardingPage>
           child: Padding(
             padding: const EdgeInsets.fromLTRB(0, 120, 0, 90),
             child: PropertyChangeConsumer<OnboardingViewModel, String>(
-              builder: (BuildContext context, BaseViewModel model, _) {
+              builder: (BuildContext context, OnboardingViewModel? model, _) {
                 return Column(
                   mainAxisSize: MainAxisSize.max,
                   mainAxisAlignment: MainAxisAlignment.start,
                   children: <Widget>[
-                    Expanded(child: _buildActiveOnboardingPage(model)),
+                    Expanded(child: _buildActiveOnboardingPage(model!)),
                     OnboardingButtonBar(
                       onPrevious: () {
                         _navigateBack(context);
@@ -101,7 +100,7 @@ class _OnboardingPageState extends State<OnboardingPage>
   }
 
   Widget _buildActiveOnboardingPage(OnboardingViewModel model) {
-    var currentStep = model.pages[model.currentStep];
+    var currentStep = model.pages[model.currentStep]!;
     var contentWidget = currentStep.buildContent(context);
 
     Widget body = Padding(
@@ -109,13 +108,11 @@ class _OnboardingPageState extends State<OnboardingPage>
       child: contentWidget,
     );
 
-    if (currentStep != null) {
-      body = PropertyChangeProvider<OnboardingStepViewModel, String>(
-        key: ValueKey(model.currentStep),
-        value: currentStep.viewModel(),
-        child: body,
-      );
-    }
+    body = PropertyChangeProvider<OnboardingStepViewModel, String>(
+      key: ValueKey(model.currentStep),
+      value: currentStep.viewModel(),
+      child: body,
+    );
 
     return IntrinsicHeight(
       child: PageTransitionSwitcher(
@@ -146,7 +143,8 @@ class _OnboardingPageState extends State<OnboardingPage>
   }
 
   void _onboardingFinished() {
-    var rootViewModel = PropertyChangeProvider.of<RootViewModel, String>(context).value;
+    var rootViewModel =
+        PropertyChangeProvider.of<RootViewModel, String>(context)!.value;
     rootViewModel.setIsOnboarding(false);
 
     Navigator.of(context).pushReplacementNamed("main");
diff --git a/lib/ui/onboarding/viewmodels/dualis_login_view_model.dart b/lib/ui/onboarding/viewmodels/dualis_login_view_model.dart
index d040074b..25e74c15 100644
--- a/lib/ui/onboarding/viewmodels/dualis_login_view_model.dart
+++ b/lib/ui/onboarding/viewmodels/dualis_login_view_model.dart
@@ -7,8 +7,7 @@ class DualisLoginViewModel extends OnboardingStepViewModel {
   final PreferencesProvider preferencesProvider;
   final DualisService dualisService;
 
-  String username;
-  String password;
+  Credentials? credentials;
 
   bool _isLoading = false;
   bool get isLoading => _isLoading;
@@ -16,27 +15,28 @@ class DualisLoginViewModel extends OnboardingStepViewModel {
   bool _loginSuccess = false;
   bool get loginSuccess => _loginSuccess;
 
-  bool _passwordOrUsernameWrong = false;
-  bool get passwordOrUsernameWrong => _passwordOrUsernameWrong;
-
   DualisLoginViewModel(this.preferencesProvider, this.dualisService);
 
-  Future<void> testCredentials(String username, String password) async {
-    this.username = username;
-    this.password = password;
+  @override
+  void setIsValid(bool isValid) {
+    if (credentials == null) {
+      super.setIsValid(false);
+    } else {
+      super.setIsValid(isValid);
+    }
+  }
 
+  Future<void> testCredentials(Credentials credentials) async {
     try {
       _isLoading = true;
 
       notifyListeners("isLoading");
 
       _loginSuccess =
-          await dualisService.login(username, password) == LoginResult.LoggedIn;
-      _passwordOrUsernameWrong = !_loginSuccess;
+          await dualisService.login(credentials) == LoginResult.LoggedIn;
       setIsValid(_loginSuccess);
     } catch (ex) {
       setIsValid(false);
-      _passwordOrUsernameWrong = true;
     } finally {
       _isLoading = false;
     }
@@ -46,13 +46,12 @@ class DualisLoginViewModel extends OnboardingStepViewModel {
 
   @override
   Future<void> save() async {
+    if (!isValid) return;
+
     await preferencesProvider.setStoreDualisCredentials(true);
 
     // To improve the onboarding experience we do not await this call because
     // it may take a few seconds
-    preferencesProvider.storeDualisCredentials(Credentials(
-      username,
-      password,
-    ));
+    preferencesProvider.storeDualisCredentials(credentials!);
   }
 }
diff --git a/lib/ui/onboarding/viewmodels/ical_url_view_model.dart b/lib/ui/onboarding/viewmodels/ical_url_view_model.dart
index 78ed03e6..346f6648 100644
--- a/lib/ui/onboarding/viewmodels/ical_url_view_model.dart
+++ b/lib/ui/onboarding/viewmodels/ical_url_view_model.dart
@@ -8,14 +8,14 @@ class IcalUrlViewModel extends OnboardingStepViewModel {
   final PreferencesProvider preferencesProvider;
   final ScheduleSourceProvider scheduleSourceProvider;
 
-  String _url;
-  String get url => _url;
+  String? _url;
+  String? get url => _url;
 
   bool urlHasError = false;
 
   IcalUrlViewModel(this.preferencesProvider, this.scheduleSourceProvider);
 
-  void setUrl(String url) {
+  void setUrl(String? url) {
     _url = url;
 
     notifyListeners("url");
@@ -24,7 +24,7 @@ class IcalUrlViewModel extends OnboardingStepViewModel {
   }
 
   void _validateUrl() {
-    urlHasError = !IcalScheduleSource.isValidUrl(_url);
+    urlHasError = !IcalScheduleSource.isValidUrl(_url!);
 
     setIsValid(!urlHasError);
 
@@ -32,10 +32,10 @@ class IcalUrlViewModel extends OnboardingStepViewModel {
   }
 
   Future<void> pasteUrl() async {
-    ClipboardData data = await Clipboard.getData('text/plain');
+    ClipboardData? data = await Clipboard.getData('text/plain');
 
     if (data?.text != null) {
-      setUrl(data.text);
+      setUrl(data!.text);
     }
   }
 
diff --git a/lib/ui/onboarding/viewmodels/mannheim_view_model.dart b/lib/ui/onboarding/viewmodels/mannheim_view_model.dart
index 757125fa..3ae7a468 100644
--- a/lib/ui/onboarding/viewmodels/mannheim_view_model.dart
+++ b/lib/ui/onboarding/viewmodels/mannheim_view_model.dart
@@ -14,11 +14,11 @@ class MannheimViewModel extends OnboardingStepViewModel {
   LoadCoursesState _loadingState = LoadCoursesState.Loading;
   LoadCoursesState get loadingState => _loadingState;
 
-  Course _selectedCourse;
-  Course get selectedCourse => _selectedCourse;
+  Course? _selectedCourse;
+  Course? get selectedCourse => _selectedCourse;
 
-  List<Course> _courses;
-  List<Course> get courses => _courses;
+  List<Course>? _courses;
+  List<Course>? get courses => _courses;
 
   MannheimViewModel(this._scheduleSourceProvider) {
     setIsValid(false);
diff --git a/lib/ui/onboarding/viewmodels/onboarding_view_model.dart b/lib/ui/onboarding/viewmodels/onboarding_view_model.dart
index 9128e541..b9749d75 100644
--- a/lib/ui/onboarding/viewmodels/onboarding_view_model.dart
+++ b/lib/ui/onboarding/viewmodels/onboarding_view_model.dart
@@ -28,11 +28,11 @@ class OnboardingViewModel extends BaseViewModel {
   final Map<String, int> stepsBackstack = {};
 
   int _stepIndex = 0;
-  int get stepIndex => _stepIndex;
+  int? get stepIndex => _stepIndex;
 
   String get currentStep => steps[_stepIndex];
 
-  bool get currentPageValid => pages[currentStep].viewModel().isValid;
+  bool get currentPageValid => pages[currentStep]!.viewModel().isValid;
   bool get isLastStep => _stepIndex >= steps.length - 1;
 
   get onboardingSteps => steps.length;
@@ -61,7 +61,7 @@ class OnboardingViewModel extends BaseViewModel {
 
     var lastPage = stepsBackstack.keys.last;
 
-    _stepIndex = stepsBackstack[lastPage];
+    _stepIndex = stepsBackstack[lastPage]!;
 
     stepsBackstack.remove(lastPage);
 
@@ -77,7 +77,7 @@ class OnboardingViewModel extends BaseViewModel {
       return;
     }
 
-    var nextDesiredStep = pages[currentStep].nextStep();
+    var nextDesiredStep = pages[currentStep]!.nextStep();
 
     stepsBackstack[currentStep] = _stepIndex;
 
@@ -96,10 +96,10 @@ class OnboardingViewModel extends BaseViewModel {
 
   Future<void> finishOnboarding() async {
     for (var step in stepsBackstack.keys) {
-      await pages[step].viewModel().save();
+      await pages[step]!.viewModel().save();
     }
 
-    _onboardingFinished?.call();
+    _onboardingFinished.call();
 
     await analytics.logTutorialComplete();
     await analytics.setUserProperty(name: "onboarding_finished", value: "true");
diff --git a/lib/ui/onboarding/viewmodels/onboarding_view_model_base.dart b/lib/ui/onboarding/viewmodels/onboarding_view_model_base.dart
index 26ebd7da..fba1ca43 100644
--- a/lib/ui/onboarding/viewmodels/onboarding_view_model_base.dart
+++ b/lib/ui/onboarding/viewmodels/onboarding_view_model_base.dart
@@ -4,6 +4,7 @@ abstract class OnboardingStepViewModel extends BaseViewModel {
   bool _isValid = false;
   bool get isValid => _isValid;
 
+  // TODO: [Leptopoda] kinda redundant now that we have null safety?¿
   void setIsValid(bool isValid) {
     _isValid = isValid;
     notifyListeners("isValid");
diff --git a/lib/ui/onboarding/viewmodels/rapla_url_view_model.dart b/lib/ui/onboarding/viewmodels/rapla_url_view_model.dart
index 76fbf4c1..6511643b 100644
--- a/lib/ui/onboarding/viewmodels/rapla_url_view_model.dart
+++ b/lib/ui/onboarding/viewmodels/rapla_url_view_model.dart
@@ -8,8 +8,8 @@ class RaplaUrlViewModel extends OnboardingStepViewModel {
   final PreferencesProvider preferencesProvider;
   final ScheduleSourceProvider scheduleSourceProvider;
 
-  String _raplaUrl;
-  String get raplaUrl => _raplaUrl;
+  String? _raplaUrl;
+  String? get raplaUrl => _raplaUrl;
 
   bool urlHasError = false;
 
@@ -18,7 +18,7 @@ class RaplaUrlViewModel extends OnboardingStepViewModel {
     this.scheduleSourceProvider,
   );
 
-  void setRaplaUrl(String url) {
+  void setRaplaUrl(String? url) {
     _raplaUrl = url;
 
     notifyListeners("raplaUrl");
@@ -27,7 +27,7 @@ class RaplaUrlViewModel extends OnboardingStepViewModel {
   }
 
   void _validateUrl() {
-    urlHasError = !RaplaScheduleSource.isValidUrl(_raplaUrl);
+    urlHasError = !RaplaScheduleSource.isValidUrl(_raplaUrl!);
 
     setIsValid(!urlHasError);
 
@@ -35,10 +35,10 @@ class RaplaUrlViewModel extends OnboardingStepViewModel {
   }
 
   Future<void> pasteUrl() async {
-    ClipboardData data = await Clipboard.getData('text/plain');
+    ClipboardData? data = await Clipboard.getData('text/plain');
 
     if (data?.text != null) {
-      setRaplaUrl(data.text);
+      setRaplaUrl(data!.text);
     }
   }
 
diff --git a/lib/ui/onboarding/viewmodels/select_source_view_model.dart b/lib/ui/onboarding/viewmodels/select_source_view_model.dart
index ec95f6fa..8310a6f3 100644
--- a/lib/ui/onboarding/viewmodels/select_source_view_model.dart
+++ b/lib/ui/onboarding/viewmodels/select_source_view_model.dart
@@ -12,9 +12,11 @@ class SelectSourceViewModel extends OnboardingStepViewModel {
     setIsValid(true);
   }
 
-  void setScheduleSourceType(ScheduleSourceType type) {
+  void setScheduleSourceType(ScheduleSourceType? type) {
+    if (type == null) return;
+
     _scheduleSourceType = type;
-    setIsValid(_scheduleSourceType != null);
+    setIsValid(true);
 
     notifyListeners("scheduleSourceType");
   }
@@ -24,7 +26,7 @@ class SelectSourceViewModel extends OnboardingStepViewModel {
     await _preferencesProvider.setScheduleSourceType(scheduleSourceType.index);
   }
 
-  String nextStep() {
+  String? nextStep() {
     switch (_scheduleSourceType) {
       case ScheduleSourceType.Rapla:
         return "rapla";
@@ -36,8 +38,8 @@ class SelectSourceViewModel extends OnboardingStepViewModel {
         return "mannheim";
       case ScheduleSourceType.Ical:
         return "ical";
+      default:
+        return null;
     }
-
-    return null;
   }
 }
diff --git a/lib/ui/onboarding/widgets/dualis_login_page.dart b/lib/ui/onboarding/widgets/dualis_login_page.dart
index 52630741..f4faf068 100644
--- a/lib/ui/onboarding/widgets/dualis_login_page.dart
+++ b/lib/ui/onboarding/widgets/dualis_login_page.dart
@@ -1,4 +1,5 @@
 import 'package:dhbwstudentapp/common/i18n/localizations.dart';
+import 'package:dhbwstudentapp/dualis/model/credentials.dart';
 import 'package:dhbwstudentapp/ui/login_credentials_widget.dart';
 import 'package:dhbwstudentapp/ui/onboarding/viewmodels/dualis_login_view_model.dart';
 import 'package:dhbwstudentapp/ui/onboarding/viewmodels/onboarding_view_model_base.dart';
@@ -10,6 +11,8 @@ import 'package:property_change_notifier/property_change_notifier.dart';
 /// credentials.
 ///
 class DualisLoginCredentialsPage extends StatefulWidget {
+  const DualisLoginCredentialsPage({Key? key}) : super(key: key);
+
   @override
   _DualisLoginCredentialsPageState createState() =>
       _DualisLoginCredentialsPageState();
@@ -17,31 +20,27 @@ class DualisLoginCredentialsPage extends StatefulWidget {
 
 class _DualisLoginCredentialsPageState
     extends State<DualisLoginCredentialsPage> {
-  final TextEditingController _usernameEditingController =
-      TextEditingController();
-  final TextEditingController _passwordEditingController =
-      TextEditingController();
+  final CredentialsEditingController _controller =
+      CredentialsEditingController();
 
   @override
   Widget build(BuildContext context) {
     return PropertyChangeConsumer<OnboardingStepViewModel, String>(
-      builder:
-          (BuildContext context, OnboardingStepViewModel base, Set<Object> _) {
-        var viewModel = base as DualisLoginViewModel;
+      builder: (BuildContext context, OnboardingStepViewModel? base,
+          Set<Object>? _) {
+        final viewModel = base as DualisLoginViewModel?;
 
-        if (_usernameEditingController.text != viewModel.username) {
-          _usernameEditingController.text = viewModel.username;
-        }
-        if (_passwordEditingController.text != viewModel.password) {
-          _passwordEditingController.text = viewModel.password;
+        final credentials = viewModel?.credentials;
+
+        if (credentials != null) {
+          _controller.credentials = credentials;
         }
 
         var widgets = <Widget>[];
 
         widgets.addAll(_buildHeader());
         widgets.add(LoginCredentialsWidget(
-          usernameEditingController: _usernameEditingController,
-          passwordEditingController: _passwordEditingController,
+          controller: _controller,
           onSubmitted: () async {
             await _testCredentials(viewModel);
           },
@@ -57,7 +56,7 @@ class _DualisLoginCredentialsPageState
     );
   }
 
-  Widget _buildTestCredentialsButton(DualisLoginViewModel viewModel) {
+  Widget _buildTestCredentialsButton(DualisLoginViewModel? viewModel) {
     return Expanded(
       child: SizedBox(
         height: 64,
@@ -69,7 +68,7 @@ class _DualisLoginCredentialsPageState
             crossAxisAlignment: CrossAxisAlignment.center,
             children: <Widget>[
               Expanded(
-                child: viewModel.passwordOrUsernameWrong
+                child: viewModel?.isValid ?? false
                     ? Text(
                         L.of(context).onboardingDualisWrongCredentials,
                         textAlign: TextAlign.start,
@@ -77,7 +76,7 @@ class _DualisLoginCredentialsPageState
                       )
                     : Container(),
               ),
-              viewModel.isLoading
+              viewModel?.isLoading ?? false
                   ? const SizedBox(
                       width: 16,
                       height: 16,
@@ -85,7 +84,7 @@ class _DualisLoginCredentialsPageState
                         strokeWidth: 1,
                       ),
                     )
-                  : viewModel.loginSuccess
+                  : viewModel?.loginSuccess ?? false
                       ? const Icon(
                           Icons.check,
                           color: Colors.green,
@@ -106,11 +105,10 @@ class _DualisLoginCredentialsPageState
     );
   }
 
-  Future _testCredentials(DualisLoginViewModel viewModel) async {
-    await viewModel.testCredentials(
-      _usernameEditingController.text,
-      _passwordEditingController.text,
-    );
+  Future _testCredentials(DualisLoginViewModel? viewModel) async {
+    if (viewModel == null) return;
+
+    await viewModel.testCredentials(_controller.credentials);
   }
 
   List<Widget> _buildHeader() {
diff --git a/lib/ui/onboarding/widgets/ical_url_page.dart b/lib/ui/onboarding/widgets/ical_url_page.dart
index dfd690bc..cdb6316f 100644
--- a/lib/ui/onboarding/widgets/ical_url_page.dart
+++ b/lib/ui/onboarding/widgets/ical_url_page.dart
@@ -5,6 +5,8 @@ import 'package:flutter/material.dart';
 import 'package:property_change_notifier/property_change_notifier.dart';
 
 class IcalUrlPage extends StatefulWidget {
+  const IcalUrlPage({Key? key}) : super(key: key);
+
   @override
   _IcalUrlPageState createState() => _IcalUrlPageState();
 }
@@ -40,12 +42,13 @@ class _IcalUrlPageState extends State<IcalUrlPage> {
           child: Padding(
             padding: const EdgeInsets.fromLTRB(0, 32, 0, 0),
             child: PropertyChangeConsumer<OnboardingStepViewModel, String>(
-              builder: (BuildContext context, OnboardingStepViewModel model,
-                  Set<Object> _) {
-                var viewModel = model as IcalUrlViewModel;
+              builder: (BuildContext context, OnboardingStepViewModel? model,
+                  Set<Object>? _) {
+                final viewModel = model as IcalUrlViewModel?;
 
-                if (_urlTextController.text != viewModel.url)
-                  _urlTextController.text = viewModel.url;
+                if (viewModel?.url != null &&
+                    _urlTextController.text != viewModel!.url)
+                  _urlTextController.text = viewModel.url!;
 
                 return Padding(
                   padding: const EdgeInsets.fromLTRB(0, 0, 0, 16),
@@ -56,19 +59,17 @@ class _IcalUrlPageState extends State<IcalUrlPage> {
                         child: TextField(
                           controller: _urlTextController,
                           decoration: InputDecoration(
-                            errorText: (viewModel.urlHasError ?? false)
+                            errorText: (viewModel?.urlHasError == true)
                                 ? L.of(context).onboardingRaplaUrlInvalid
                                 : null,
                             hintText: L.of(context).onboardingIcalUrlHint,
                           ),
-                          onChanged: (value) {
-                            viewModel.setUrl(value);
-                          },
+                          onChanged: viewModel?.setUrl,
                         ),
                       ),
                       TextButton.icon(
                         onPressed: () async {
-                          await viewModel.pasteUrl();
+                          await viewModel?.pasteUrl();
                         },
                         icon: Icon(Icons.content_paste),
                         label: Text(
diff --git a/lib/ui/onboarding/widgets/mannheim_page.dart b/lib/ui/onboarding/widgets/mannheim_page.dart
index 83495596..ff9c0890 100644
--- a/lib/ui/onboarding/widgets/mannheim_page.dart
+++ b/lib/ui/onboarding/widgets/mannheim_page.dart
@@ -5,6 +5,8 @@ import 'package:flutter/material.dart';
 import 'package:property_change_notifier/property_change_notifier.dart';
 
 class MannheimPage extends StatefulWidget {
+  const MannheimPage({Key? key}) : super(key: key);
+
   @override
   _MannheimPageState createState() => _MannheimPageState();
 }
@@ -50,20 +52,19 @@ class SelectMannheimCourseWidget extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     return PropertyChangeConsumer<OnboardingStepViewModel, String>(
-      builder:
-          (BuildContext context, OnboardingStepViewModel model, Set<Object> _) {
-        var viewModel = model as MannheimViewModel;
+      builder: (BuildContext context, OnboardingStepViewModel? model,
+          Set<Object>? _) {
+        final viewModel = model as MannheimViewModel?;
 
-        switch (viewModel.loadingState) {
+        switch (viewModel?.loadingState) {
           case LoadCoursesState.Loading:
             return _buildLoadingIndicator();
           case LoadCoursesState.Loaded:
-            return _buildCourseList(context, viewModel);
+            return _buildCourseList(context, viewModel!);
           case LoadCoursesState.Failed:
+          default:
             return _buildLoadingError(context, viewModel);
         }
-
-        return Container();
       },
     );
   }
@@ -89,7 +90,8 @@ class SelectMannheimCourseWidget extends StatelessWidget {
     int index,
     BuildContext context,
   ) {
-    var isSelected = viewModel.selectedCourse == viewModel.courses[index];
+    // TODO: [Leptopoda] why is nullsafety garanttueed here but checked above ¿?
+    var isSelected = viewModel.selectedCourse == viewModel.courses![index];
 
     return ListTile(
       trailing: isSelected
@@ -99,19 +101,20 @@ class SelectMannheimCourseWidget extends StatelessWidget {
             )
           : null,
       title: Text(
-        viewModel.courses[index].name,
+        viewModel.courses![index].name,
         style: isSelected
             ? TextStyle(
                 color: Theme.of(context).colorScheme.secondary,
               )
             : null,
       ),
-      subtitle: Text(viewModel.courses[index].title),
-      onTap: () => viewModel.setSelectedCourse(viewModel.courses[index]),
+      subtitle: Text(viewModel.courses![index].title),
+      onTap: () => viewModel.setSelectedCourse(viewModel.courses![index]),
     );
   }
 
-  Widget _buildLoadingError(BuildContext context, MannheimViewModel viewModel) {
+  Widget _buildLoadingError(
+      BuildContext context, MannheimViewModel? viewModel) {
     return Center(
       child: Column(
         mainAxisAlignment: MainAxisAlignment.center,
@@ -120,7 +123,7 @@ class SelectMannheimCourseWidget extends StatelessWidget {
           Padding(
             padding: const EdgeInsets.all(16),
             child: MaterialButton(
-              onPressed: viewModel.loadCourses,
+              onPressed: viewModel?.loadCourses,
               child: Icon(Icons.refresh),
             ),
           ),
diff --git a/lib/ui/onboarding/widgets/onboarding_button_bar.dart b/lib/ui/onboarding/widgets/onboarding_button_bar.dart
index 41f4deb6..f95cc7ab 100644
--- a/lib/ui/onboarding/widgets/onboarding_button_bar.dart
+++ b/lib/ui/onboarding/widgets/onboarding_button_bar.dart
@@ -4,14 +4,14 @@ import 'package:flutter/material.dart';
 
 class OnboardingButtonBar extends StatelessWidget {
   final OnboardingViewModel viewModel;
-  final Function onNext;
-  final Function onPrevious;
+  final VoidCallback onNext;
+  final VoidCallback onPrevious;
 
   const OnboardingButtonBar({
-    Key key,
-    @required this.viewModel,
-    @required this.onNext,
-    @required this.onPrevious,
+    Key? key,
+    required this.viewModel,
+    required this.onNext,
+    required this.onPrevious,
   }) : super(key: key);
 
   @override
diff --git a/lib/ui/onboarding/widgets/onboarding_page_background.dart b/lib/ui/onboarding/widgets/onboarding_page_background.dart
index 715b0747..b31b0242 100644
--- a/lib/ui/onboarding/widgets/onboarding_page_background.dart
+++ b/lib/ui/onboarding/widgets/onboarding_page_background.dart
@@ -20,7 +20,7 @@ class OnboardingPageBackground extends StatelessWidget {
     Brightness.dark: "assets/onboarding_bottom_background_dark.png",
   };
 
-  OnboardingPageBackground({Key key, this.controller})
+  OnboardingPageBackground({Key? key, required this.controller})
       : angleTopBackground = Tween<double>(
           begin: PlatformUtil.isPhone() ? -32 : -10,
           end: PlatformUtil.isPhone() ? -10 : -5,
@@ -75,7 +75,7 @@ class OnboardingPageBackground extends StatelessWidget {
         ),
         super(key: key);
 
-  Widget _buildAnimation(BuildContext context, Widget child) {
+  Widget _buildAnimation(BuildContext context, Widget? child) {
     return Stack(
       children: <Widget>[
         Transform.rotate(
@@ -111,7 +111,7 @@ class OnboardingPageBackground extends StatelessWidget {
                     child: Transform.scale(
                       scale: 1.5,
                       child: Image.asset(
-                        background[Theme.of(context).brightness],
+                        background[Theme.of(context).brightness]!,
                       ),
                     ),
                   ),
@@ -129,7 +129,7 @@ class OnboardingPageBackground extends StatelessWidget {
                     child: Transform.scale(
                       scale: 1.5,
                       child: Image.asset(
-                        foreground[Theme.of(context).brightness],
+                        foreground[Theme.of(context).brightness]!,
                       ),
                     ),
                   ),
diff --git a/lib/ui/onboarding/widgets/rapla_url_page.dart b/lib/ui/onboarding/widgets/rapla_url_page.dart
index 2e5b5f3e..512b5d12 100644
--- a/lib/ui/onboarding/widgets/rapla_url_page.dart
+++ b/lib/ui/onboarding/widgets/rapla_url_page.dart
@@ -5,6 +5,8 @@ import 'package:flutter/material.dart';
 import 'package:property_change_notifier/property_change_notifier.dart';
 
 class RaplaUrlPage extends StatefulWidget {
+  const RaplaUrlPage({Key? key}) : super(key: key);
+
   @override
   _RaplaUrlPageState createState() => _RaplaUrlPageState();
 }
@@ -40,12 +42,13 @@ class _RaplaUrlPageState extends State<RaplaUrlPage> {
           child: Padding(
             padding: const EdgeInsets.fromLTRB(0, 32, 0, 0),
             child: PropertyChangeConsumer<OnboardingStepViewModel, String>(
-              builder: (BuildContext context, OnboardingStepViewModel model,
-                  Set<Object> _) {
-                var viewModel = model as RaplaUrlViewModel;
+              builder: (BuildContext context, OnboardingStepViewModel? model,
+                  Set<Object>? _) {
+                final viewModel = model as RaplaUrlViewModel?;
 
-                if (_urlTextController.text != viewModel.raplaUrl)
-                  _urlTextController.text = viewModel.raplaUrl;
+                if (viewModel?.raplaUrl != null &&
+                    _urlTextController.text != viewModel!.raplaUrl)
+                  _urlTextController.text = viewModel.raplaUrl!;
 
                 return Padding(
                   padding: const EdgeInsets.fromLTRB(0, 0, 0, 16),
@@ -56,19 +59,17 @@ class _RaplaUrlPageState extends State<RaplaUrlPage> {
                         child: TextField(
                           controller: _urlTextController,
                           decoration: InputDecoration(
-                            errorText: (viewModel.urlHasError ?? false)
+                            errorText: viewModel?.urlHasError == true
                                 ? L.of(context).onboardingRaplaUrlInvalid
                                 : null,
                             hintText: L.of(context).onboardingRaplaUrlHint,
                           ),
-                          onChanged: (value) {
-                            viewModel.setRaplaUrl(value);
-                          },
+                          onChanged: viewModel?.setRaplaUrl,
                         ),
                       ),
                       TextButton.icon(
                         onPressed: () async {
-                          await viewModel.pasteUrl();
+                          await viewModel?.pasteUrl();
                         },
                         icon: Icon(Icons.content_paste),
                         label: Text(
diff --git a/lib/ui/onboarding/widgets/select_source_page.dart b/lib/ui/onboarding/widgets/select_source_page.dart
index e10eb9e5..b420a366 100644
--- a/lib/ui/onboarding/widgets/select_source_page.dart
+++ b/lib/ui/onboarding/widgets/select_source_page.dart
@@ -6,17 +6,17 @@ import 'package:flutter/material.dart';
 import 'package:property_change_notifier/property_change_notifier.dart';
 
 class SelectSourcePage extends StatelessWidget {
-  const SelectSourcePage({Key key}) : super(key: key);
+  const SelectSourcePage({Key? key}) : super(key: key);
 
   @override
   Widget build(BuildContext context) {
     return PropertyChangeConsumer<OnboardingStepViewModel, String>(
       builder: (
         BuildContext context,
-        OnboardingStepViewModel model,
-        Set<Object> _,
+        OnboardingStepViewModel? model,
+        Set<Object>? _,
       ) {
-        var viewModel = model as SelectSourceViewModel;
+        final viewModel = model as SelectSourceViewModel?;
         return Column(
           mainAxisAlignment: MainAxisAlignment.start,
           children: <Widget>[
@@ -92,15 +92,15 @@ class SelectSourcePage extends StatelessWidget {
   }
 
   RadioListTile<ScheduleSourceType> buildScheduleTypeRadio(
-      SelectSourceViewModel viewModel,
+      SelectSourceViewModel? viewModel,
       BuildContext context,
       ScheduleSourceType type,
       String title) {
     return RadioListTile(
       value: type, //model.useDualis,
-      onChanged: viewModel.setScheduleSourceType,
+      onChanged: viewModel?.setScheduleSourceType,
       title: Text(title),
-      groupValue: viewModel.scheduleSourceType,
+      groupValue: viewModel?.scheduleSourceType,
     );
   }
 }
diff --git a/lib/ui/pager_widget.dart b/lib/ui/pager_widget.dart
index 96cbc68f..7d2dc3ed 100644
--- a/lib/ui/pager_widget.dart
+++ b/lib/ui/pager_widget.dart
@@ -15,24 +15,20 @@ import 'package:provider/provider.dart';
 ///
 class PagerWidget extends StatefulWidget {
   final List<PageDefinition> pages;
-  final String pagesId;
+  final String? pagesId;
 
-  const PagerWidget({Key key, @required this.pages, this.pagesId})
+  const PagerWidget({Key? key, required this.pages, this.pagesId})
       : super(key: key);
 
   @override
-  _PagerWidgetState createState() => _PagerWidgetState(pages, pagesId);
+  _PagerWidgetState createState() => _PagerWidgetState();
 }
 
 class _PagerWidgetState extends State<PagerWidget> {
   final PreferencesProvider preferencesProvider = KiwiContainer().resolve();
 
-  final String pagesId;
-  final List<PageDefinition> pages;
   int _currentPage = 0;
 
-  _PagerWidgetState(this.pages, this.pagesId);
-
   @override
   void initState() {
     super.initState();
@@ -49,37 +45,23 @@ class _PagerWidgetState extends State<PagerWidget> {
           key: ValueKey(_currentPage),
           children: <Widget>[
             Expanded(
-              child: _wrapWithChangeNotifierProvider(
-                pages[_currentPage].builder(context),
-                pages[_currentPage].viewModel,
-              ),
+              child: widget.pages[_currentPage].widget(context),
             ),
           ],
         ),
       ),
       bottomNavigationBar: BottomNavigationBar(
         currentIndex: _currentPage,
-        onTap: (int index) async {
-          await setActivePage(index);
-        },
+        onTap: setActivePage,
         items: buildBottomNavigationBarItems(),
       ),
     );
   }
 
-  Widget _wrapWithChangeNotifierProvider(Widget child, BaseViewModel value) {
-    if (value == null) return child;
-
-    return ChangeNotifierProvider.value(
-      child: child,
-      value: value,
-    );
-  }
-
   List<BottomNavigationBarItem> buildBottomNavigationBarItems() {
     var bottomNavigationBarItems = <BottomNavigationBarItem>[];
 
-    for (var page in pages) {
+    for (var page in widget.pages) {
       bottomNavigationBarItems.add(
         BottomNavigationBarItem(
           icon: page.icon,
@@ -91,7 +73,7 @@ class _PagerWidgetState extends State<PagerWidget> {
   }
 
   Future<void> setActivePage(int page) async {
-    if (page < 0 || page >= pages.length) {
+    if (page < 0 || page >= widget.pages.length) {
       return;
     }
 
@@ -99,20 +81,20 @@ class _PagerWidgetState extends State<PagerWidget> {
       _currentPage = page;
     });
 
-    if (pagesId == null) return;
-    await preferencesProvider.set("${pagesId}_active_page", page);
+    if (widget.pagesId == null) return;
+    await preferencesProvider.set("${widget.pagesId}_active_page", page);
   }
 
   Future<void> loadActivePage() async {
-    if (pagesId == null) return;
+    if (widget.pagesId == null) return;
 
     var selectedPage = await preferencesProvider.get<int>(
-      "${pagesId}_active_page",
+      "${widget.pagesId}_active_page",
     );
 
     if (selectedPage == null) return;
 
-    if (selectedPage > 0 && selectedPage < pages.length) {
+    if (selectedPage > 0 && selectedPage < widget.pages.length) {
       setState(() {
         _currentPage = selectedPage;
       });
@@ -120,16 +102,26 @@ class _PagerWidgetState extends State<PagerWidget> {
   }
 }
 
-class PageDefinition {
+class PageDefinition<T extends BaseViewModel> {
   final Widget icon;
   final String text;
   final WidgetBuilder builder;
-  final BaseViewModel viewModel;
+  final T? viewModel;
 
   PageDefinition({
-    @required this.icon,
-    @required this.text,
-    @required this.builder,
+    required this.icon,
+    required this.text,
+    required this.builder,
     this.viewModel,
   });
+
+  /// Wraps the Widget with a [ChangeNotifierProvider] if [viewModel] is specified.
+  Widget widget(BuildContext context) {
+    if (viewModel == null) return builder(context);
+
+    return ChangeNotifierProvider.value(
+      child: builder(context),
+      value: viewModel,
+    );
+  }
 }
diff --git a/lib/ui/root_page.dart b/lib/ui/root_page.dart
index 82397d57..cb6c2027 100644
--- a/lib/ui/root_page.dart
+++ b/lib/ui/root_page.dart
@@ -16,17 +16,13 @@ import 'package:property_change_notifier/property_change_notifier.dart';
 class RootPage extends StatefulWidget {
   final RootViewModel rootViewModel;
 
-  const RootPage({Key key, this.rootViewModel}) : super(key: key);
+  const RootPage({Key? key, required this.rootViewModel}) : super(key: key);
 
   @override
-  _RootPageState createState() => _RootPageState(rootViewModel);
+  _RootPageState createState() => _RootPageState();
 }
 
 class _RootPageState extends State<RootPage> {
-  final RootViewModel rootViewModel;
-
-  _RootPageState(this.rootViewModel);
-
   @override
   void initState() {
     super.initState();
@@ -37,10 +33,12 @@ class _RootPageState extends State<RootPage> {
     return PropertyChangeProvider<RootViewModel, String>(
       child: PropertyChangeConsumer(
         properties: const ["appTheme", "isOnboarding"],
-        builder: (BuildContext context, RootViewModel model, Set properties) =>
-            MaterialApp(
-          theme: ColorPalettes.buildTheme(model.appTheme),
-          initialRoute: rootViewModel.isOnboarding ? "onboarding" : "main",
+        builder:
+            (BuildContext context, RootViewModel? model, Set? properties) =>
+                MaterialApp(
+          theme: ColorPalettes.buildTheme(model!.appTheme),
+          initialRoute:
+              widget.rootViewModel.isOnboarding ? "onboarding" : "main",
           navigatorKey: NavigatorKey.rootKey,
           navigatorObservers: [rootNavigationObserver],
           localizationsDelegates: [
@@ -57,7 +55,7 @@ class _RootPageState extends State<RootPage> {
           onGenerateRoute: generateRoute,
         ),
       ),
-      value: rootViewModel,
+      value: widget.rootViewModel,
     );
   }
 }
diff --git a/lib/ui/settings/donate_list_tile.dart b/lib/ui/settings/donate_list_tile.dart
index 9562b533..4b17d511 100644
--- a/lib/ui/settings/donate_list_tile.dart
+++ b/lib/ui/settings/donate_list_tile.dart
@@ -7,6 +7,8 @@ import 'package:kiwi/kiwi.dart';
 import 'package:property_change_notifier/property_change_notifier.dart';
 
 class DonateListTile extends StatefulWidget {
+  const DonateListTile({Key? key}) : super(key: key);
+
   @override
   _DonateListTileState createState() => _DonateListTileState();
 }
@@ -14,7 +16,7 @@ class DonateListTile extends StatefulWidget {
 class _DonateListTileState extends State<DonateListTile> {
   final InAppPurchaseManager inAppPurchaseManager;
 
-  SettingsViewModel model;
+  late SettingsViewModel model;
 
   bool isPurchasing = false;
 
@@ -34,7 +36,7 @@ class _DonateListTileState extends State<DonateListTile> {
     inAppPurchaseManager.removePurchaseCallback(null, purchaseCallback);
   }
 
-  void purchaseCallback(String productId, PurchaseResultEnum result) {
+  void purchaseCallback(String? productId, PurchaseResultEnum result) {
     if (!mounted) return;
 
     setState(() {
@@ -49,7 +51,8 @@ class _DonateListTileState extends State<DonateListTile> {
       properties: const [
         "didPurchaseWidget",
       ],
-    ).value;
+    )!
+        .value;
 
     if (model.widgetPurchaseState == null) {
       return Padding(
@@ -91,8 +94,8 @@ class _DonateListTileState extends State<DonateListTile> {
   }
 
   void _purchaseClicked() async {
-    if (isPurchasing || model.widgetPurchaseState == PurchaseStateEnum.Purchased)
-      return;
+    if (isPurchasing ||
+        model.widgetPurchaseState == PurchaseStateEnum.Purchased) return;
 
     setState(() {
       isPurchasing = true;
diff --git a/lib/ui/settings/purchase_widget_list_tile.dart b/lib/ui/settings/purchase_widget_list_tile.dart
index 8f8d9438..5cbd5594 100644
--- a/lib/ui/settings/purchase_widget_list_tile.dart
+++ b/lib/ui/settings/purchase_widget_list_tile.dart
@@ -7,13 +7,15 @@ import 'package:kiwi/kiwi.dart';
 import 'package:property_change_notifier/property_change_notifier.dart';
 
 class PurchaseWidgetListTile extends StatefulWidget {
+  const PurchaseWidgetListTile({Key? key}) : super(key: key);
+
   @override
   _PurchaseWidgetListTileState createState() => _PurchaseWidgetListTileState();
 }
 
 class _PurchaseWidgetListTileState extends State<PurchaseWidgetListTile> {
   final InAppPurchaseManager inAppPurchaseManager;
-  SettingsViewModel model;
+  late SettingsViewModel model;
 
   bool isPurchasing = false;
 
@@ -34,7 +36,7 @@ class _PurchaseWidgetListTileState extends State<PurchaseWidgetListTile> {
     inAppPurchaseManager.removePurchaseCallback(null, purchaseCallback);
   }
 
-  void purchaseCallback(String productId, PurchaseResultEnum result) {
+  void purchaseCallback(String? productId, PurchaseResultEnum result) {
     if (!mounted) return;
 
     setState(() {
@@ -50,9 +52,10 @@ class _PurchaseWidgetListTileState extends State<PurchaseWidgetListTile> {
         "didPurchaseWidget",
         "areWidgetsSupported",
       ],
-    ).value;
+    )!
+        .value;
 
-    if (!model.areWidgetsSupported ||
+    if (!model.areWidgetsSupported! ||
         model.widgetPurchaseState == PurchaseStateEnum.Unknown ||
         model.widgetPurchaseState == null) {
       return Container();
@@ -66,7 +69,7 @@ class _PurchaseWidgetListTileState extends State<PurchaseWidgetListTile> {
     );
   }
 
-  Widget _buildWidgetSubtitle() {
+  Widget? _buildWidgetSubtitle() {
     if (isPurchasing) {
       return Align(
         alignment: Alignment.centerLeft,
diff --git a/lib/ui/settings/select_theme_dialog.dart b/lib/ui/settings/select_theme_dialog.dart
index c6a337c1..340711b9 100644
--- a/lib/ui/settings/select_theme_dialog.dart
+++ b/lib/ui/settings/select_theme_dialog.dart
@@ -28,7 +28,8 @@ class SelectThemeDialog {
           properties: const [
             "appTheme",
           ],
-          builder: (BuildContext context, RootViewModel model, Set properties) {
+          builder:
+              (BuildContext context, RootViewModel? model, Set? properties) {
             return Column(
               mainAxisSize: MainAxisSize.min,
               children: [
@@ -36,19 +37,19 @@ class SelectThemeDialog {
                   title: Text(L.of(context).selectThemeLight),
                   value: AppTheme.Light,
                   groupValue: _rootViewModel.appTheme,
-                  onChanged: (v) => _rootViewModel.setAppTheme(v),
+                  onChanged: _rootViewModel.setAppTheme,
                 ),
                 RadioListTile(
                   title: Text(L.of(context).selectThemeDark),
                   value: AppTheme.Dark,
                   groupValue: _rootViewModel.appTheme,
-                  onChanged: (v) => _rootViewModel.setAppTheme(v),
+                  onChanged: _rootViewModel.setAppTheme,
                 ),
                 RadioListTile(
                   title: Text(L.of(context).selectThemeSystem),
                   value: AppTheme.System,
                   groupValue: _rootViewModel.appTheme,
-                  onChanged: (v) => _rootViewModel.setAppTheme(v),
+                  onChanged: _rootViewModel.setAppTheme,
                 ),
               ],
             );
diff --git a/lib/ui/settings/settings_page.dart b/lib/ui/settings/settings_page.dart
index 7c7c173a..4d966bda 100644
--- a/lib/ui/settings/settings_page.dart
+++ b/lib/ui/settings/settings_page.dart
@@ -30,6 +30,8 @@ import 'package:url_launcher/url_launcher.dart';
 /// of the app
 ///
 class SettingsPage extends StatefulWidget {
+  const SettingsPage({Key? key}) : super(key: key);
+
   @override
   _SettingsPageState createState() => _SettingsPageState();
 }
@@ -128,10 +130,10 @@ class _SettingsPageState extends State<SettingsPage> {
           "prettifySchedule",
         ],
         builder:
-            (BuildContext context, SettingsViewModel model, Set properties) {
+            (BuildContext context, SettingsViewModel? model, Set? properties) {
           return SwitchListTile(
             title: Text(L.of(context).settingsPrettifySchedule),
-            onChanged: model.setPrettifySchedule,
+            onChanged: model!.setPrettifySchedule,
             value: model.prettifySchedule,
           );
         },
@@ -162,7 +164,7 @@ class _SettingsPageState extends State<SettingsPage> {
               .isCalendarSyncEnabled();
           List<DateEntry> entriesToExport =
               KiwiContainer().resolve<ListDateEntries30d>().listDateEntries;
-          await NavigatorKey.rootKey.currentState.push(MaterialPageRoute(
+          await NavigatorKey.rootKey.currentState!.push(MaterialPageRoute(
               builder: (BuildContext context) => CalendarExportPage(
                     entriesToExport: entriesToExport,
                     isCalendarSyncWidget: true,
@@ -177,18 +179,18 @@ class _SettingsPageState extends State<SettingsPage> {
 
   List<Widget> buildNotificationSettings(BuildContext context) {
     WorkSchedulerService service = KiwiContainer().resolve();
-    if (service?.isSchedulingAvailable() ?? false) {
+    if (service.isSchedulingAvailable()) {
       return [
         TitleListTile(title: L.of(context).settingsNotificationsTitle),
         PropertyChangeConsumer(
           properties: const [
             "notifyAboutNextDay",
           ],
-          builder:
-              (BuildContext context, SettingsViewModel model, Set properties) {
+          builder: (BuildContext context, SettingsViewModel? model,
+              Set? properties) {
             return SwitchListTile(
               title: Text(L.of(context).settingsNotificationsNextDay),
-              onChanged: model.setNotifyAboutNextDay,
+              onChanged: model!.setNotifyAboutNextDay,
               value: model.notifyAboutNextDay,
             );
           },
@@ -197,11 +199,11 @@ class _SettingsPageState extends State<SettingsPage> {
           properties: const [
             "notifyAboutScheduleChanges",
           ],
-          builder:
-              (BuildContext context, SettingsViewModel model, Set properties) {
+          builder: (BuildContext context, SettingsViewModel? model,
+              Set? properties) {
             return SwitchListTile(
               title: Text(L.of(context).settingsNotificationsScheduleChange),
-              onChanged: model.setNotifyAboutScheduleChanges,
+              onChanged: model!.setNotifyAboutScheduleChanges,
               value: model.notifyAboutScheduleChanges,
             );
           },
@@ -220,17 +222,17 @@ class _SettingsPageState extends State<SettingsPage> {
         properties: const [
           "appTheme",
         ],
-        builder: (BuildContext context, RootViewModel model, Set properties) {
+        builder: (BuildContext context, RootViewModel? model, Set? properties) {
           return ListTile(
             title: Text(L.of(context).settingsDarkMode),
             onTap: () async {
-              await SelectThemeDialog(model).show(context);
+              await SelectThemeDialog(model!).show(context);
             },
             subtitle: Text({
               AppTheme.Dark: L.of(context).selectThemeDark,
               AppTheme.Light: L.of(context).selectThemeLight,
               AppTheme.System: L.of(context).selectThemeSystem,
-            }[model.appTheme]),
+            }[model!.appTheme]!),
           );
         },
       ),
diff --git a/lib/ui/settings/viewmodels/settings_view_model.dart b/lib/ui/settings/viewmodels/settings_view_model.dart
index 9db30d99..7bffb54c 100644
--- a/lib/ui/settings/viewmodels/settings_view_model.dart
+++ b/lib/ui/settings/viewmodels/settings_view_model.dart
@@ -34,20 +34,19 @@ class SettingsViewModel extends BaseViewModel {
 
   bool get isCalendarSyncEnabled => _isCalendarSyncEnabled;
 
-  PurchaseStateEnum _widgetPurchaseState;
+  PurchaseStateEnum? _widgetPurchaseState;
 
-  PurchaseStateEnum get widgetPurchaseState => _widgetPurchaseState;
+  PurchaseStateEnum? get widgetPurchaseState => _widgetPurchaseState;
 
-  bool _areWidgetsSupported = false;
+  bool? _areWidgetsSupported = false;
 
-  bool get areWidgetsSupported => _areWidgetsSupported;
+  bool? get areWidgetsSupported => _areWidgetsSupported;
 
   SettingsViewModel(
-    this._preferencesProvider,
-    this._nextDayInformationNotification,
-    this._widgetHelper,
-    this._inAppPurchaseManager
-  ) {
+      this._preferencesProvider,
+      this._nextDayInformationNotification,
+      this._widgetHelper,
+      this._inAppPurchaseManager) {
     _loadPreferences();
 
     _inAppPurchaseManager.addPurchaseCallback(
@@ -71,7 +70,7 @@ class SettingsViewModel extends BaseViewModel {
     );
   }
 
-  void _widgetPurchaseCallback(String id, PurchaseResultEnum result) {
+  void _widgetPurchaseCallback(String? id, PurchaseResultEnum result) {
     if (result == PurchaseResultEnum.Success) {
       _widgetPurchaseState = PurchaseStateEnum.Purchased;
     }
diff --git a/pubspec.lock b/pubspec.lock
index f05222ab..7a882907 100644
--- a/pubspec.lock
+++ b/pubspec.lock
@@ -22,6 +22,13 @@ packages:
       url: "https://pub.dartlang.org"
     source: hosted
     version: "2.0.2"
+  app_group_directory:
+    dependency: "direct main"
+    description:
+      name: app_group_directory
+      url: "https://pub.dartlang.org"
+    source: hosted
+    version: "2.0.0"
   archive:
     dependency: transitive
     description:
@@ -42,7 +49,7 @@ packages:
       name: async
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "2.8.2"
+    version: "2.9.0"
   boolean_selector:
     dependency: transitive
     description:
@@ -56,7 +63,7 @@ packages:
       name: characters
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.2.0"
+    version: "1.2.1"
   charcode:
     dependency: transitive
     description:
@@ -70,7 +77,7 @@ packages:
       name: clock
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.1.0"
+    version: "1.1.1"
   collection:
     dependency: transitive
     description:
@@ -120,13 +127,20 @@ packages:
       url: "https://pub.dartlang.org"
     source: hosted
     version: "4.2.0"
+  equatable:
+    dependency: "direct main"
+    description:
+      name: equatable
+      url: "https://pub.dartlang.org"
+    source: hosted
+    version: "2.0.5"
   fake_async:
     dependency: transitive
     description:
       name: fake_async
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.3.0"
+    version: "1.3.1"
   ffi:
     dependency: transitive
     description:
@@ -378,13 +392,6 @@ packages:
       url: "https://pub.dartlang.org"
     source: hosted
     version: "1.0.3"
-  ios_app_group:
-    dependency: "direct main"
-    description:
-      name: ios_app_group
-      url: "https://pub.dartlang.org"
-    source: hosted
-    version: "1.0.2"
   js:
     dependency: transitive
     description:
@@ -426,21 +433,21 @@ packages:
       name: matcher
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "0.12.11"
+    version: "0.12.12"
   material_color_utilities:
     dependency: transitive
     description:
       name: material_color_utilities
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "0.1.4"
+    version: "0.1.5"
   meta:
     dependency: transitive
     description:
       name: meta
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.7.0"
+    version: "1.8.0"
   mime:
     dependency: transitive
     description:
@@ -482,7 +489,7 @@ packages:
       name: path
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.8.1"
+    version: "1.8.2"
   path_provider:
     dependency: "direct main"
     description:
@@ -697,7 +704,7 @@ packages:
       name: source_span
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.8.2"
+    version: "1.9.0"
   sprintf:
     dependency: transitive
     description:
@@ -739,7 +746,7 @@ packages:
       name: string_scanner
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.1.0"
+    version: "1.1.1"
   synchronized:
     dependency: transitive
     description:
@@ -753,28 +760,28 @@ packages:
       name: term_glyph
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.2.0"
+    version: "1.2.1"
   test:
     dependency: "direct dev"
     description:
       name: test
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.21.1"
+    version: "1.21.4"
   test_api:
     dependency: transitive
     description:
       name: test_api
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "0.4.9"
+    version: "0.4.12"
   test_core:
     dependency: transitive
     description:
       name: test_core
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "0.4.13"
+    version: "0.4.16"
   timezone:
     dependency: "direct main"
     description:
@@ -907,7 +914,7 @@ packages:
       name: workmanager
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "0.4.1"
+    version: "0.5.0"
   xdg_directories:
     dependency: transitive
     description:
@@ -930,5 +937,5 @@ packages:
     source: hosted
     version: "3.1.0"
 sdks:
-  dart: ">=2.17.0-0 <3.0.0"
+  dart: ">=2.17.0 <3.0.0"
   flutter: ">=2.10.0"
diff --git a/pubspec.yaml b/pubspec.yaml
index 18ed687b..3f04227c 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -14,7 +14,7 @@ description: An app for the DHBW Stuttgart
 version: 1.0.1+1
 
 environment:
-  sdk: ">=2.10.0 <3.0.0"
+  sdk: ">=2.12.0 <3.0.0"
 
 dependencies:
   flutter:
@@ -35,7 +35,7 @@ dependencies:
   url_launcher: ^6.1.0
   kiwi: ^4.0.2
   flutter_local_notifications: ^9.4.1 # BREAKING CHANGES HERE!
-  workmanager: ^0.4.1
+  workmanager: ^0.5.0
   universal_html: ^2.0.8
   firebase_core: ^1.15.0
   firebase_analytics: ^9.1.6
@@ -48,8 +48,9 @@ dependencies:
   device_calendar: ^4.2.0
   flutter_inapp_purchase: ^5.1.2
   flutter_widgetkit: ^1.0.3
-  ios_app_group: ^1.0.0
+  app_group_directory: ^2.0.0
   timezone: ^0.8.0
+  equatable: ^2.0.0
 
 dev_dependencies:
   flutter_test:
@@ -57,13 +58,11 @@ dev_dependencies:
 
   test: any
 
-
 # For information on the generic Dart part of this file, see the
 # following page: https://dart.dev/tools/pub/pubspec
 
 # The following section is specific to Flutter.
 flutter:
-
   # The following line ensures that the Material Icons font is
   # included with your application, so that you can use the icons in
   # the material Icons class.
@@ -81,6 +80,6 @@ flutter:
     - assets/schedule_empty_state_dark.png
 
   fonts:
-    - family:  CustomIcons
+    - family: CustomIcons
       fonts:
         - asset: fonts/CustomIcons.ttf
diff --git a/test/dualis/service/parsing/modules_from_course_result_page_extract_test.dart b/test/dualis/service/parsing/modules_from_course_result_page_extract_test.dart
index 28bcc7d9..fd347f36 100644
--- a/test/dualis/service/parsing/modules_from_course_result_page_extract_test.dart
+++ b/test/dualis/service/parsing/modules_from_course_result_page_extract_test.dart
@@ -17,12 +17,12 @@ Future<void> main() async {
 
     expect(modules.length, 3);
 
-    expect(modules[1].id, "T3INF1001");
-    expect(modules[1].name, "Mathematik I");
-    expect(modules[1].state, null);
-    expect(modules[1].credits, "8,0");
-    expect(modules[1].finalGrade, "4,0");
-    expect(modules[1].detailsUrl,
+    expect(modules[1]!.id, "T3INF1001");
+    expect(modules[1]!.name, "Mathematik I");
+    expect(modules[1]!.state, null);
+    expect(modules[1]!.credits, "8,0");
+    expect(modules[1]!.finalGrade, "4,0");
+    expect(modules[1]!.detailsUrl,
         "www.endpoint.com/scripts/mgrqispi.dll?APPNAME=CampusNet&PRGNAME=RESULTDETAILS&ARGUMENTS=-N123456789876543,-N000307,-N121212121212121");
   });
 
diff --git a/test/schedule/service/rapla/rapla_response_parser_test.dart b/test/schedule/service/rapla/rapla_response_parser_test.dart
index ced0b81d..deda5af1 100644
--- a/test/schedule/service/rapla/rapla_response_parser_test.dart
+++ b/test/schedule/service/rapla/rapla_response_parser_test.dart
@@ -114,7 +114,8 @@ Future<void> main() async {
     expect(schedule.entries[0].end, DateTime(2021, 09, 22, 15, 00));
     expect(schedule.entries[0].type, ScheduleEntryType.Class);
     expect(schedule.entries[0].professor, "A");
-    expect(schedule.entries[0].room, "MOS-TINF19A,A 1.380 Vorlesungsraum (Mi 22.09.21  08:00, Do 23.09.21  08:00),A 1.390 Vorlesungsraum (Mi 22.09.21  08:00, Do 23.09.21  08:00)");
+    expect(schedule.entries[0].room,
+        "MOS-TINF19A,A 1.380 Vorlesungsraum (Mi 22.09.21  08:00, Do 23.09.21  08:00),A 1.390 Vorlesungsraum (Mi 22.09.21  08:00, Do 23.09.21  08:00)");
 
     expect(schedule.entries[2].title, "Tag der Deutschen Einheit");
     expect(schedule.entries[2].start, DateTime(2021, 10, 03, 08, 00));
@@ -165,8 +166,7 @@ Future<void> main() async {
     expect(schedule.entries.length, 20);
   });
 
-  test('Rapla correctly read all classes of several months view 3',
-      () async {
+  test('Rapla correctly read all classes of several months view 3', () async {
     var parser = RaplaResponseParser();
 
     var schedule = parser.parseSchedule(severalMonthsPage2).schedule;
@@ -179,8 +179,7 @@ Future<void> main() async {
     expect(schedule.entries.length, 36);
   });
 
-  test('Rapla correctly read the day of a class in week view',
-      () async {
+  test('Rapla correctly read the day of a class in week view', () async {
     var parser = RaplaResponseParser();
 
     var schedule = parser.parseSchedule(raplaPage1).schedule;
@@ -190,55 +189,63 @@ Future<void> main() async {
     expect(schedule.entries[0].end, DateTime(2021, 11, 02, 12, 15));
     expect(schedule.entries[0].type, ScheduleEntryType.Class);
     expect(schedule.entries[0].professor, "Fr, Ta");
-    expect(schedule.entries[0].room, "WDCM21B,G086 W Hörsaal (Di 26.10.21  13:00, Do 04.11.21  12:45, Di 16.11.21  10:00),A167 W Hörsaal (Di 30.11.21  10:00),F218_1 PA Hörsaal (Di 07.12.21  10:00, Mi 08.12.21  13:00),XOnline-Veranstaltung A Virtueller Raum (Di 02.11.21  09:00, Do 11.11.21  09:00)");
+    expect(schedule.entries[0].room,
+        "WDCM21B,G086 W Hörsaal (Di 26.10.21  13:00, Do 04.11.21  12:45, Di 16.11.21  10:00),A167 W Hörsaal (Di 30.11.21  10:00),F218_1 PA Hörsaal (Di 07.12.21  10:00, Mi 08.12.21  13:00),XOnline-Veranstaltung A Virtueller Raum (Di 02.11.21  09:00, Do 11.11.21  09:00)");
 
-    expect(schedule.entries[1].title, "Einführung in die Volkswirtschaftslehre und Mikroökonomik");
+    expect(schedule.entries[1].title,
+        "Einführung in die Volkswirtschaftslehre und Mikroökonomik");
     expect(schedule.entries[1].start, DateTime(2021, 11, 02, 13, 45));
     expect(schedule.entries[1].end, DateTime(2021, 11, 02, 17, 00));
     expect(schedule.entries[1].type, ScheduleEntryType.Class);
     expect(schedule.entries[1].professor, "Le, An");
-    expect(schedule.entries[1].room, "WDCM21B,D221 W Hörsaal (Mo 11.10.21  09:00),G086 W Hörsaal (Mo 25.10.21  09:00, Mo 15.11.21  09:00),F218_1 PA Hörsaal (Mo 22.11.21  09:00, Mo 06.12.21  09:00),A167 W Hörsaal (Mo 29.11.21  09:00),XOnline-Veranstaltung A Virtueller Raum (Di 02.11.21  13:45)");
+    expect(schedule.entries[1].room,
+        "WDCM21B,D221 W Hörsaal (Mo 11.10.21  09:00),G086 W Hörsaal (Mo 25.10.21  09:00, Mo 15.11.21  09:00),F218_1 PA Hörsaal (Mo 22.11.21  09:00, Mo 06.12.21  09:00),A167 W Hörsaal (Mo 29.11.21  09:00),XOnline-Veranstaltung A Virtueller Raum (Di 02.11.21  13:45)");
 
     expect(schedule.entries[2].title, "Grundlagen des Bürgerlichen Rechts");
     expect(schedule.entries[2].start, DateTime(2021, 11, 03, 09, 00));
     expect(schedule.entries[2].end, DateTime(2021, 11, 03, 11, 30));
     expect(schedule.entries[2].type, ScheduleEntryType.Class);
     expect(schedule.entries[2].professor, "Ei, An");
-    expect(schedule.entries[2].room, "WDCM21B,XOnline-Veranstaltung A Virtueller Raum (Mi 13.10.21  09:00, Mi 27.10.21  09:00, Mi 10.11.21  09:00, Mi 24.11.21  09:00, Fr 03.12.21  10:00),D221 W Hörsaal (Mi 20.10.21  13:00, Mi 17.11.21  09:00),G086 W Hörsaal (Di 26.10.21  09:00, Mi 03.11.21  09:00),B354 W Hörsaal (Fr 03.12.21  10:00),F218_1 PA Hörsaal (Mi 08.12.21  09:00)");
+    expect(schedule.entries[2].room,
+        "WDCM21B,XOnline-Veranstaltung A Virtueller Raum (Mi 13.10.21  09:00, Mi 27.10.21  09:00, Mi 10.11.21  09:00, Mi 24.11.21  09:00, Fr 03.12.21  10:00),D221 W Hörsaal (Mi 20.10.21  13:00, Mi 17.11.21  09:00),G086 W Hörsaal (Di 26.10.21  09:00, Mi 03.11.21  09:00),B354 W Hörsaal (Fr 03.12.21  10:00),F218_1 PA Hörsaal (Mi 08.12.21  09:00)");
 
     expect(schedule.entries[3].title, "Technik der Finanzbuchführung I");
     expect(schedule.entries[3].start, DateTime(2021, 11, 03, 13, 00));
     expect(schedule.entries[3].end, DateTime(2021, 11, 03, 16, 15));
     expect(schedule.entries[3].type, ScheduleEntryType.Class);
     expect(schedule.entries[3].professor, "Se, Ka");
-    expect(schedule.entries[3].room, "WDCM21B,D221 W Hörsaal (Mi 17.11.21  13:00, Mi 06.10.21  13:00, Mi 13.10.21  13:00),G086 W Hörsaal (Mi 27.10.21  13:00, Mi 03.11.21  13:00, Mi 10.11.21  13:00),A167 W Hörsaal (Mi 24.11.21  13:00, Mi 01.12.21  14:00)");
+    expect(schedule.entries[3].room,
+        "WDCM21B,D221 W Hörsaal (Mi 17.11.21  13:00, Mi 06.10.21  13:00, Mi 13.10.21  13:00),G086 W Hörsaal (Mi 27.10.21  13:00, Mi 03.11.21  13:00, Mi 10.11.21  13:00),A167 W Hörsaal (Mi 24.11.21  13:00, Mi 01.12.21  14:00)");
 
-    expect(schedule.entries[4].title, "Grundlagen des wissenschaftlichen Arbeitens");
+    expect(schedule.entries[4].title,
+        "Grundlagen des wissenschaftlichen Arbeitens");
     expect(schedule.entries[4].start, DateTime(2021, 11, 04, 09, 00));
     expect(schedule.entries[4].end, DateTime(2021, 11, 04, 12, 15));
     expect(schedule.entries[4].type, ScheduleEntryType.Class);
     expect(schedule.entries[4].professor, "He, Be");
-    expect(schedule.entries[4].room, "WDCM21B,D221 W Hörsaal (Di 05.10.21  09:00, Di 12.10.21  09:00),A167 W Hörsaal (Di 23.11.21  09:00),G086 W Hörsaal (Do 04.11.21  09:00)");
+    expect(schedule.entries[4].room,
+        "WDCM21B,D221 W Hörsaal (Di 05.10.21  09:00, Di 12.10.21  09:00),A167 W Hörsaal (Di 23.11.21  09:00),G086 W Hörsaal (Do 04.11.21  09:00)");
 
     expect(schedule.entries[5].title, "Grundlagen der Handelsbetriebslehre");
     expect(schedule.entries[5].start, DateTime(2021, 11, 04, 12, 45));
     expect(schedule.entries[5].end, DateTime(2021, 11, 04, 16, 00));
     expect(schedule.entries[5].type, ScheduleEntryType.Class);
     expect(schedule.entries[5].professor, "Fr, Ta");
-    expect(schedule.entries[5].room, "WDCM21B,G086 W Hörsaal (Di 26.10.21  13:00, Do 04.11.21  12:45, Di 16.11.21  10:00),A167 W Hörsaal (Di 30.11.21  10:00),F218_1 PA Hörsaal (Di 07.12.21  10:00, Mi 08.12.21  13:00),XOnline-Veranstaltung A Virtueller Raum (Di 02.11.21  09:00, Do 11.11.21  09:00)");
+    expect(schedule.entries[5].room,
+        "WDCM21B,G086 W Hörsaal (Di 26.10.21  13:00, Do 04.11.21  12:45, Di 16.11.21  10:00),A167 W Hörsaal (Di 30.11.21  10:00),F218_1 PA Hörsaal (Di 07.12.21  10:00, Mi 08.12.21  13:00),XOnline-Veranstaltung A Virtueller Raum (Di 02.11.21  09:00, Do 11.11.21  09:00)");
 
     expect(schedule.entries[6].title, "Einführung in die Programmierung");
     expect(schedule.entries[6].start, DateTime(2021, 11, 05, 13, 00));
     expect(schedule.entries[6].end, DateTime(2021, 11, 05, 16, 15));
     expect(schedule.entries[6].type, ScheduleEntryType.Class);
     expect(schedule.entries[6].professor, "He, Ma");
-    expect(schedule.entries[6].room, "WDCM21B,C348  PC Raum,D221 W Hörsaal (Fr 08.10.21  13:00, Fr 15.10.21  13:00, Fr 22.10.21  13:00),G086 W Hörsaal (Fr 29.10.21  13:00, Fr 05.11.21  13:00, Fr 12.11.21  13:00, Do 18.11.21  13:00),A167 W Hörsaal (Fr 26.11.21  13:00, Do 02.12.21  13:00)");
+    expect(schedule.entries[6].room,
+        "WDCM21B,C348  PC Raum,D221 W Hörsaal (Fr 08.10.21  13:00, Fr 15.10.21  13:00, Fr 22.10.21  13:00),G086 W Hörsaal (Fr 29.10.21  13:00, Fr 05.11.21  13:00, Fr 12.11.21  13:00, Do 18.11.21  13:00),A167 W Hörsaal (Fr 26.11.21  13:00, Do 02.12.21  13:00)");
 
     expect(schedule.entries.length, 7);
   });
 
-  test('Rapla correctly read the week response',
-      () async {
+  test('Rapla correctly read the week response', () async {
     var parser = RaplaResponseParser();
 
     var schedule = parser.parseSchedule(raplaWeekResponse).schedule;
@@ -253,8 +260,7 @@ Future<void> main() async {
     expect(schedule.entries.length, 6);
   });
 
-  test('Rapla correctly read the week response 1',
-      () async {
+  test('Rapla correctly read the week response 1', () async {
     var parser = RaplaResponseParser();
 
     var schedule = parser.parseSchedule(raplaWeekResponse1).schedule;
@@ -264,7 +270,8 @@ Future<void> main() async {
     expect(schedule.entries[0].end, DateTime(2021, 12, 13, 10, 00));
     expect(schedule.entries[0].type, ScheduleEntryType.Exam);
     expect(schedule.entries[0].professor, "Man, R.");
-    expect(schedule.entries[0].room, "TEA20,H031, Hörsaal,N003, Hörsaal,N004, Hörsaal");
+    expect(schedule.entries[0].room,
+        "TEA20,H031, Hörsaal,N003, Hörsaal,N004, Hörsaal");
 
     expect(schedule.entries.length, 7);
   });