takePriority method Null safety
Make this BackButtonDispatcher take priority among its peers.
This has no effect when a BackButtonDispatcher has no parents and no children. If a BackButtonDispatcher does have parents or children, however, it causes this object to be the one to dispatch the notification when the parent would normally notify its callback.
The BackButtonDispatcher must have a listener registered before it can be told to take priority.
void takePriority() => _children.clear();