Fixes infinite loop in NavPolygonInstance warnings

scene/2d/navigation_polygon.cpp

@@ -452,7 +452,7 @@ String NavigationPolygonInstance::get_configuration_warning() const {
 			return String();
 		}
 
-		c = Object::cast_to<Node2D>(get_parent());
+		c = Object::cast_to<Node2D>(c->get_parent());
 	}
 
 	return TTR("NavigationPolygonInstance must be a child or grandchild to a Navigation2D node. It only provides navigation data.");