Fix adding a NavigationPolygonInstance to a Node2D freezing the editor

Fixes #11975

scene/2d/navigation_polygon.cpp

@@ -329,7 +329,7 @@ void NavigationPolygonInstance::_notification(int p_what) {
 					break;
 				}
 
-				c = Object::cast_to<Node2D>(get_parent());
+				c = Object::cast_to<Node2D>(c->get_parent());
 			}
 
 		} break;