protected function getCombinedUrl($outputFilename = 'undefined.css')
    {
        $combineAction = 'System\Classes\Controller@combine';
        $actionExists = Route::getRoutes()->getByAction($combineAction) !== null;

        if ($actionExists) {
            return Url::action($combineAction, [$outputFilename], false);
        }

        return '/combine/'.$outputFilename;
    }