/**
  * Return the URL to the admin panel page.
  *
  * Return the URL to the admin panel page.
  *
  * @since 5.0.0 
  * @access static
  *
  * @return string URL to the admin panel page.
  */
 public static function get_url()
 {
     $url = self::$url;
     if (empty($url)) {
         if (is_multisite()) {
             $url = add_query_arg(array('page' => self::get_slug()), network_admin_url('index.php'));
         } else {
             $url = add_query_arg(array('page' => self::get_slug()), admin_url('index.php'));
         }
         self::$url = $url;
     }
     return $url;
 }