onRemovedFromStage property Null safety
Dispatched when a display object is about to be removed from the display list, either directly or through the removal of a sub tree in which the display object is contained.
Two methods of the DisplayObjectContainer class generate this event:
The following methods of a DisplayObjectContainer object also generate this event if an object must be removed to make room for the object:
EventStream<Event> get onRemovedFromStage => DisplayObject.removedFromStageEvent.forTarget(this);