Replace fatal errors in transitionDuration delegate methods (#642)

This PR modifies the transitionDuration(using:) method in FloatingPanelController to return 0.0 instead of calling fatalError when the FloatingPanelController instance is not found. This change is crucial for several reasons:

1. Avoiding Crashes in Production: Using fatalError in production can lead to unexpected crashes, which are detrimental to user experience. It's safer to return a default value like 0.0 and handle the scenario gracefully.

2. Improved Stability: By returning 0.0, the application can continue running, allowing for better stability and user satisfaction. This approach also aligns with the principle of fail-safety, where the system continues operating under error conditions.

3. Real-World Case: In our large-scale project, we encountered a crash at this specific point due to the fatalError. Given the size and complexity of our application, it has been challenging to pinpoint the exact cause. Switching to a return value of 0.0 would significantly help us mitigate the issue and maintain app stability while we investigate further.

4. Maintainability: Returning a default value makes the codebase more maintainable and easier to debug, as it avoids abrupt termination and allows for logging or other error handling mechanisms.

Sources/Transitioning.swift

@@ -84,7 +84,7 @@ class ModalPresentTransition: NSObject, UIViewControllerAnimatedTransitioning {
     func transitionDuration(using transitionContext: UIViewControllerContextTransitioning?) -> TimeInterval {
         guard
             let fpc = transitionContext?.viewController(forKey: .to) as? FloatingPanelController
-        else { fatalError()}
+        else { return 0.0 }
 
         let animator = fpc.animatorForPresenting(to: fpc.layout.initialState)
         return TimeInterval(animator.duration)
@@ -119,7 +119,7 @@ class ModalDismissTransition: NSObject, UIViewControllerAnimatedTransitioning {
     func transitionDuration(using transitionContext: UIViewControllerContextTransitioning?) -> TimeInterval {
         guard
             let fpc = transitionContext?.viewController(forKey: .from) as? FloatingPanelController
-        else { fatalError()}
+        else { return 0.0 }
 
         let animator = fpc.animatorForDismissing(with: .zero)
         return TimeInterval(animator.duration)