public function flushMessage(){
        if( session_id() ){
            foreach( [ $this->error_key => 'error', $this->message_key => 'updated' ] as $key => $class_name ){
                if( isset($_SESSION[$key]) && !empty($_SESSION[$key]) ){
                    $markup = sprintf('<div class="%s"><p>%s</p></div>', $class_name, implode('<br />', $_SESSION[$key]));
                    if( is_admin() ){
                        echo $markup;
                    }else{
                        /**
                         * Post redirect get message's filter
                         *
                         * @param string $markup html string
                         * @param array $messages Messages' array
                         * @param string $class_name updated or error.
                         * @return string
                         */
                        echo apply_filters('wpametu_prg_message_class', $markup, $_SESSION[$key], $class_name);
                    }
                    // Make session empty.
                    $_SESSION[$key] = [];
                }
            }
        }
    }