Called when the draggable is dropped and accepted by a DragTarget.
This function might be called after this widget has been removed from the tree. For example, if a drag was in progress when this widget was removed from the tree and the drag ended up completing, this callback will still be called. For this reason, implementations of this callback might need to check State.mounted to check whether the state receiving the callback is still in the tree.
final VoidCallback onDragCompleted