Flutter bloc - emit was called after an event handler completed normally

Question

I have this code in user_bloc.dart:

UserBloc(this._userRepository, UserState userState) : super(userState) {
    on<RegisterUser>(_onRegisterUser);
  }

void _onRegisterUser(RegisterUser event, Emitter<UserState> emit) async {
    emit(UserRegistering());
    try {
      // detect when the user is registered
      FirebaseAuth.instance.authStateChanges().listen((User? user) async {
        if (state is UserRegistering && user != null) {
          // save user to db
          try {
            await _userRepository.addUpdateUserInfo(user.uid, userInfo);
          } catch (e) {
            emit(UserRegisteringError("Could not save user"));
          }
          emit(UserLoggedIn(user));
        }
      });
       ... call FirebaseAuth.instance.createUserWithEmailAndPassword

this is _userRepository.addUpdateUserInfo:

Future<void> addUpdateUserInfo(String userId, UserInfo userInfo) async {
    try {
      var doc = usersInfo.doc(userId);
      await doc.set(
        {
          'first_name': userInfo.firstName,
          'last_name': userInfo.lastName,
          'email': userInfo.email
        },
        SetOptions(merge: true),
      );
    } catch (e) {
      print("Failed to add user: $e");
      throw Exception("Failed to add user");
    }
  }

When emit(UserLoggedIn(user)); is called I get this error:

Error: Assertion failed:
..\…\src\emitter.dart:114
!_isCompleted
"\n\n\nemit was called after an event handler completed normally.\nThis is usually due to an unawaited future in an event handler.\nPlease make sure to await all asynchronous operations with event handlers\nand use emit.isDone after asynchronous operations before calling emit() to\nensure the event handler has not completed.\n\n  **BAD**\n  on<Event>((event, emit) {\n    future.whenComplete(() => emit(...));\n  });\n\n  **GOOD**\n  on<Event>((event, emit) async {\n    await future.whenComplete(() => emit(...));\n  });\n"
    at Object.throw_ [as throw] (http://localhost:58334/dart_sdk.js:5067:11)
    at Object.assertFailed (http://localhost:58334/dart_sdk.js:4992:15)
at _Emitter.new.call (http://localhost:58334/packages/bloc/src/transition.dart.lib.js:765:40)
at user_bloc.UserBloc.new.<anonymous> (http://localhost:58334/packages/soli/blocs/bloc/user_bloc.dart.lib.js:90:20)
    at Generator.next (<anonymous>)
    at http://localhost:58334/dart_sdk.js:40571:33
    at _RootZone.runUnary (http://localhost:58334/dart_sdk.js:40441:59)
    at _FutureListener.thenAwait.handleValue (http://localhost:58334/dart_sdk.js:35363:29)
    at handleValueCallback (http://localhost:58334/dart_sdk.js:35931:49)
    at Function._propagateToListeners (http://localhost:58334/dart_sdk.js:35969:17)
    at _Future.new.[_completeWithValue] (http://localhost:58334/dart_sdk.js:35817:23)
    at async._AsyncCallbackEntry.new.callback (http://localhost:58334/dart_sdk.js:35838:35)
    at Object._microtaskLoop (http://localhost:58334/dart_sdk.js:40708:13)
    at _startMicrotaskLoop (http://localhost:58334/dart_sdk.js:40714:13)
    at http://localhost:58334/dart_sdk.js:36191:9
[2022-02-17T03:47:00.057Z]  @firebase/firestore:

After I get the error, if I try again with a different user, it works OK.