Dart StreamController TimeoutException after 0:00:30.000000: Test timed out after 30 seconds

I'm trying to create a reactive SharedPreferences utility but I'm stuck with this issue. This is my class

class SPUtil {
    final _workoutsStreamController = StreamController<
          Result<Iterable<PreferencesWorkout>, Exception>>.broadcast();

    @override
    Stream<Result<Iterable<PreferencesWorkout>, Exception>> getWorkouts() async* {
      final prefs = await SharedPreferences.getInstance();
      _workoutsStreamController.sink.add(success(_getStoredWorkouts(prefs)));
      yield* _workoutsStreamController.stream;
    }
}

And this is my test

test("getWorkouts SHOULD return empty list WHEN nothing is stored",
        () async {
      SharedPreferences.setMockInitialValues({});

      final actual = await _sut.getWorkouts().first;

      expect((actual as Success).value, []);
    });

Whenever I run this test it loops for 30 seconds and it returns this error

dart:async  _startMicrotaskLoop

TimeoutException after 0:00:30.000000: Test timed out after 30 seconds. See https://pub.dev/packages/test#timeouts

Everything works fine if I use this implementation instead

class SPUtil {
    @override
    Stream<Result<Iterable<PreferencesWorkout>, Exception>> getWorkouts() async* {
      final prefs = await SharedPreferences.getInstance();
      yield success(_getStoredWorkouts(prefs));
    }
}

So I assume my test is correct. Thanks in advance.