Invalid sitemap name in code

$is_exist = file_exists(\Drupal::service('file_system')->realpath(file_default_scheme() . "://") . '/google_image_sitemap/sitemap_' . $gis_obj->node_type . '.xml');

in this code you are checking the sitemap fiel with sitemap_' . $gis_obj->node_type . '.xml

But when you generate the sitemap file with different name.

if ($result->node_type != 'all') {
        $query->condition('nfd.type', $result->node_type);
        $filename = 'google_image_sitemap_' . $result->node_type . '.xml';
      }