Flutter: A chain of two streams

streams.dart

class ProfileModel extends ChangeNotifier {
  final Stream<User?> _userStream = FirebaseAuth.instance.authStateChanges();
  User? user;
  Stream<DocumentSnapshot<Profile>>? _profileStream;
  Profile? profile;

  ProfileModel() {
    _userStream.listen((User? newUser) {
      user = newUser;
      profile = null;
      if (user != null) {
        _profileStream = FirebaseFirestore.instance
            .collection('profiles')
            .withConverter(
              fromFirestore: Profile.fromFirestore,
              toFirestore: (Profile profile, options) => profile.toFirestore(),
            )
            .doc(user!.uid)
            .snapshots();
        _profileStream!.listen((event) {
          profile = event.data();
          notifyListeners();
        });
      }
      notifyListeners();
    });
  }
}