public function toHTML($custom_template = null)
    {
        $custom_template = is_null($custom_template)
            ? 'laravel-notifications/'.config('laravel-notifications.view')
            : $custom_template;

        if (view()->exists($custom_template)) {
            return view($custom_template, ['messages' => $this->filtered])->render();
        } else {
            return view('laravel-notifications::bootstrap3', ['messages' => $this->filtered])->render();
        }
    }