Removed navigation_mesh instantiation from constructor of NavigationPolygon

scene/resources/navigation_polygon.cpp

@@ -503,6 +503,4 @@ void NavigationPolygon::_validate_property(PropertyInfo &p_property) const {
 	}
 }
 
-NavigationPolygon::NavigationPolygon() {
-	navigation_mesh.instantiate();
-}
+NavigationPolygon::NavigationPolygon() {}