public function checkElementKey(ServerRequest $request, Response $response): Response
    {
        $elementKey = $request->getQueryParams()['key'];

        $objectManager = GeneralUtility::makeInstance(ObjectManager::class);
        $storageRepository = $objectManager->get(StorageRepository::class);

        $isAvailable = true;
        if ($storageRepository->loadElement('tt_content', $elementKey)) {
            $isAvailable = false;
        }

        return new JsonResponse(['isAvailable' => $isAvailable]);
    }