Fix potential crash due to invalid navigation mesh ref

Fixes potential crash due to invalid navigation mesh ref, e.g. due to NavigationPolygon call unref() internally while the resource is still in the update queue of the server.

modules/navigation/nav_region.cpp

@@ -84,11 +84,11 @@ void NavRegion::set_transform(Transform3D p_transform) {
 
 void NavRegion::set_navigation_mesh(Ref<NavigationMesh> p_navigation_mesh) {
 #ifdef DEBUG_ENABLED
-	if (map && !Math::is_equal_approx(double(map->get_cell_size()), double(p_navigation_mesh->get_cell_size()))) {
+	if (map && p_navigation_mesh.is_valid() && !Math::is_equal_approx(double(map->get_cell_size()), double(p_navigation_mesh->get_cell_size()))) {
 		ERR_PRINT_ONCE(vformat("Attempted to update a navigation region with a navigation mesh that uses a `cell_size` of %s while assigned to a navigation map set to a `cell_size` of %s. The cell size for navigation maps can be changed by using the NavigationServer map_set_cell_size() function. The cell size for default navigation maps can also be changed in the ProjectSettings.", double(p_navigation_mesh->get_cell_size()), double(map->get_cell_size())));
 	}
 
-	if (map && !Math::is_equal_approx(double(map->get_cell_height()), double(p_navigation_mesh->get_cell_height()))) {
+	if (map && p_navigation_mesh.is_valid() && !Math::is_equal_approx(double(map->get_cell_height()), double(p_navigation_mesh->get_cell_height()))) {
 		ERR_PRINT_ONCE(vformat("Attempted to update a navigation region with a navigation mesh that uses a `cell_height` of %s while assigned to a navigation map set to a `cell_height` of %s. The cell height for navigation maps can be changed by using the NavigationServer map_set_cell_height() function. The cell height for default navigation maps can also be changed in the ProjectSettings.", double(p_navigation_mesh->get_cell_height()), double(map->get_cell_height())));
 	}
 #endif // DEBUG_ENABLED