Fix crash when deleting the player in AnimationPlayerEditorPlugin

[Chaosus]

Fix #82549

The player is just a weak pointer, which doesn't allocate and shouldn't free in this class:

player = Object::cast_to<AnimationPlayer>(p_object);

[TokageItLab]

I have not been able to test this because I am exactly away from home, but I don't think this fix will be a major problem since it is a destructor behavior change. In any case, crashes should be avoided and should be fixed later if this causes regressions.

It would be helpful if someone could test just to see if the dummy player pointer is broken or not when using AnimationTree.

[akien-mga]

Thanks!