I've finally taken the plunge to upgrade my app to Null Safety, but am having problems upgrading the following code.

(If not obvious, I'm mapping the Firebase User model to a local AppUser model, and using this as authentication wrapper around the entire app.)

```
class AuthService with ChangeNotifier {
  final FirebaseAuth _auth = FirebaseAuth.instance;
  late AuthResultStatus _authStatus;

  Stream<AppUser> get appUser {
    return _auth.userChanges().map(_appUserFromFirebaseUser);
  }

AppUser _appUserFromFirebaseUser(User user) {
    return user != null
        ? AppUser(
            uid: user.uid,
            creationTime: user.metadata.creationTime,
            email: user.email,
            emailVerified: user.emailVerified,
          )
        : null;
  }
}

class AppUser {
  final String? uid;
  final DateTime? creationTime;
  final String? email;
  final bool? emailVerified;

  AppUser({
    this.uid,
    this.creationTime,
    this.email,
    this.emailVerified,
  });
}

```

How do I return either a local AppUser model or null? Appreciate your pointers.