Why are child widget StreamBuilders not receiving errors?

Question

I am struggling with RxDart (maybe just straight up Rx programming). I currently have a stateful widget that calls my bloc in it's didChangeDependencies(). That call goes out and gets data via http request and adds it to a stream. I'm using BehaviorSubject and this works fine. I have child widgets using StreamBuilders and they get data no problem. My issue comes in dealing with errors. If my http request fails, I hydrate the stream with addError('whatever error') but my child widget's StreamBuilder is not receiving that error. It doesn't get anything at all.

So I have a few questions.

1. Is that expected?
2. Should error handling not be done in StreamBuilder? Ideally, I want to show something in the UI if something goes wrong so not sure how else to do it.
3. I could make my child widget stateful and use stream.listen. I do receive the errors there but it seems like overkill to have that and the StreamBuilder.
4. Am I missing something fundamental here about streams and error handling?

Here is my bloc:

final _plans = BehaviorSubject<List<PlanModel>>();
Observable<List<PlanModel>> get plans => _plans.stream;

fetchPlans() async {
    try {
        final _plans = await _planRepository.getPlans();
        _plans.add(_plans);
    }
    on AuthenticationException {
        _plans.addError('authentication error');
    }
    on SocketException {
        _plans.addError('no network connection');
    }
    catch(error) {
        _plans.addError('fetch unsuccessful');
    }
}

Simplified Parent Widget:

class PlanPage extends StatefulWidget {
  @override
  PlanPageState createState() {
    return new PlanPageState();
  }
}

class PlanPageState extends State<PlanPage> {

  @override
  void didChangeDependencies() async {
    super.didChangeDependencies();
    var planBloc = BaseProvider.of<PlanBloc>(context);
    planBloc.fetchPlans();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar( title: const Text('Your Plan') ),
        body: PlanWrapper()
    );
  }
}

Simplified Child Widget with StreamBuilder:

class PlanWrapper extends StatelessWidget {

  @override
  Widget build(BuildContext context) {
    var planBloc = BaseProvider.of<PlanBloc>(context);

    return StreamBuilder(
      stream: planBloc.plans,
      builder: (BuildContext context, AsyncSnapshot<List<PlanModel>> plans) {
        if (plans.hasError) {
          //ERROR NEVER COMES IN HERE
          switch(plans.error) {
            case 'authentication error':
              return RyAuthErrorCard();
            case 'no network connection':
              return RyNetworkErrorCard();
            default: 
              return RyGenericErrorCard(GeneralException().errorMessages()['message']);
          }
        }
        if (plans.hasData && plans.data.isNotEmpty) {
          return ListView(
            physics: const AlwaysScrollableScrollPhysics(),
            children: _buildPlanTiles(context, plans.data)
          );
        }
        return Center(child: const CircularProgressIndicator());
      }
    );
  }
}

Solution

There was an issue about this in the RxDart GitHub (https://github.com/ReactiveX/rxdart/issues/227). BehaviorSubjects were not replaying errors to new listeners.

It was fixed in version 0.21.0. "Breaking Change: BehaviorSubject will now emit an Error, if the last event was also an Error. Before, when an Error occurred before a listen, the subscriber would not be notified of that Error."