addNode method Null safety

Implementation

@override
void addNode(AsyncNode node) {
  if (isClosed) {
    return _emitEvent(QueueEventType.violateAddWhenClosed);
  }

  _enqueue(node);

  if (_map.containsKey(node.label)) {
    throw DuplicatedLabelException("A job with this label already exists");
  }
  _map[node.label] = node;
  _enqueue(node);

  if (_autoRun) start();
}