resume method Null safety

Implementation

void resume() {
  if (_state == SMHTaskState.waiting ||
      _state == SMHTaskState.processing ||
      _state == SMHTaskState.success) {
    return;
  }

  _state = SMHTaskState.waiting;
  stateStream.add(_state);
  this.getTaskInfo().cancelToken = CancelToken();
  SMHTaskManager.instance.addTask(this);
}