protected function getFrontendGroups()
    {
        $frontendGroupsList = implode(',', $this->frontendGroups);
        $frontendGroups = GeneralUtility::trimExplode(',', $frontendGroupsList,
            true);

        // clean up: filter double groups
        $frontendGroups = array_unique($frontendGroups);
        $frontendGroups = array_values($frontendGroups);

        if (empty($frontendGroups)) {
            // most likely an empty page with no content elements => public
            $frontendGroups[] = '0';
        }

        return $frontendGroups;
    }