public function add_notification( $id, $content, $type = null, $hide_button = null, $cond_func = null ) {
        if ( isset( $this->notices[$id] ) ) {
            return false;
        }

        if ( !$type ) {
            $type = 'success';
        }

        if ( !$hide_button ) {
            $hide_button = true;
        }

        if ( !$cond_func ) {
            $cond_func = false;
        }

        $this->notices[$id] = array(
            'id' => $id,
            'content' => $content,
            'type' => $type,
            'hide_button' => $hide_button,
            'cond_func' => $cond_func,
        );

        $this->save();
    }