Fixes crash in Spatial::notification

scene/3d/spatial.cpp

@@ -125,6 +125,7 @@ void Spatial::_notification(int p_what) {
 
 	switch (p_what) {
 		case NOTIFICATION_ENTER_TREE: {
+			ERR_FAIL_COND(!get_tree());
 
 			Node *p = get_parent();
 			if (p)