onRemove method
Called right before the component is removed from the game.
This method will only run for a component that was previously mounted into a component tree. If a component was never mounted (for example, when it is removed before it had a chance to mount), then this callback will not trigger. Thus, onRemove runs if and only if there was a corresponding onMount call before.
Implementation
@override
void onRemove() {
_world = null;
}