[UIKit, SwiftUI] Document-based app hangs when document initialization throws exception

Steps to reproduce:

1. Create a default document-based app for iOS using SwiftUI or UIKit with UIDocumentViewController.
2. In the document loading method, simulate an error by throwing an exception, for example: throw CocoaError(.coderValueNotFound)
3. Open the app on device/simulator running iOS 18 and attempt to open or create a new document.

Expected behavior: An appropriate error message should be displayed to the user.

Actual behavior:

• No error message is shown to the user
• The "Create Document" button becomes permanently disabled
• The app appears to hang or become unresponsive

Environment:

• iOS 18, 26 beta 9
• Both UIKit and SwiftUI implementations affected

Has anyone encountered this issue or found a workaround? This seems like a regression in iOS 18's document handling.

FB20189617, FB20189669