Update field_bloc.dart

[mrchenmo]

When modifying emit, _stateController may have been closed and an error will be thrown. All emit methods should check whether the isClosed value has been reached before calling.

void onEmit(State state) {
          if (isClosed) return;
          if (this.state == state && _emitted) return;
          onTransition(
            Transition(
              currentState: this.state,
              event: event as E,
              nextState: state,
            ),
          );
          emit(state);
        }

All emit methods in the source code will check the value of isClosed before calling it.

[richard457]

does this fix this #338 @mrchenmo ?