Unable to bindstream from firestore .snapshot().listen

[CSharper63]

Hi guys,

The app : Manage movie and who is currently watching the movie.

I'm facing of a problem. I try since 2 days to bind a stream from firestore query using a Rx<List> but I m unable to populate my list. I already test every lines of the following requests everything is working fine. Something goes wrong when try .bindstream().

My code :
My database.dart (file to interact with firestore)

Future<Stream<List<UserModel>>> viewersStream(String movieId) async {
    List<String> viewersIds = [];
    List<UserModel> viewers = [];

    await _firestore
        .collection(MOVIES_COLLECTION)
        .doc(movieId)
        .collection(VIEWERS_COLLECTION)
        .get()
        .then((value) {
      value.docs.forEach((element) {
        viewersIds.add(element.id);
      });

      return _firestore
          .collection(USERS_COLLECTION)
          .where(FieldPath.documentId, whereIn: viewersIds)
          .snapshots()
          .listen((event) {
        event.docChanges.forEach((element) {
          print("try adding to return list user: " + element.doc.id); // works
          viewers.add(UserModel.fromDocumentSnapshot(
              documentSnapshot: element.doc));
          print("user add to return list user: " + element.doc.id); // works
        });
        return viewers;
      });
    });
  }

then my movie_controller.dart :

void fetchingViewers(String movieId) async {
    if (movieId != null || movieId != "") {
      print("fetching viewers for movie " + movieId);
      _participants.bindStream(await Database().viewersStream(movieId));
      print("fetched viewers for movie " + movieId);
    }
  }

call stack error:

I hope someone could help me.