static function get_instance()
 {
     if (!self::$instance) {
         $klass = get_class();
         self::$instance = new $klass();
     }
     return self::$instance;
 }
 /**
  * Shortcode handler for [ngg_pr_proofing]
  *
  * @return mixed
  */
 function render_proofed_images()
 {
     $controller = C_NextGen_Pro_Proofing_Controller::get_instance();
     return $controller->index_action();
 }