protected function buildNotifier(Notifier $notifier = null)
    {
        $class = config('lern.notify.class');
        $class = !empty($class) ? $class : Notifier::class;
        if (empty($notifier)) {
            $notifier = new $class();
        }
        if ($notifier instanceof Notifier) {
            return $notifier;
        } else {
            throw new NotifierFailedException("LERN was expecting an instance of ".Notifier::class);
        }
    }