Open Radar

 
I have attached a sample project demonstrating the use of `ModelContext.registeredModel(for:)` to get already-fetched models. The method crashes when the generic type does not match the actual type as shown in the sample project:
```
private func track(for trackID: PersistentIdentifier) -> Track? {
   // Crashes when `registeredModel` tries to force cast a `MusicVideo` instance to `Song`.
   if let song: Song = modelContext.registeredModel(for: trackID) {
      return .song(song)
   }

   if let musicVideo: MusicVideo = modelContext.registeredModel(for: trackID) {
      return .musicVideo(musicVideo)
   }

   return nil
}
```

`ModelContext.registeredModel(for:)` tries to force cast the model instance to the generic type. This is unnecessary since the method returns an optional. Rather than crashing, the method should just return `nil` if the type did not match by using a conditional cast instead of a force cast.