Exception when page end date is in the past

[dmitryd]

When the page end date is in the past clicking on the "Form export" module shows exception regardless of the selected page in the page tree.

Problem happens because Frappant\FrpFormAnswers\Utility\BackendUtility::filterPagesForAccess() calls $page = $pageRepository->getPage($pid); but does not validate that it returns the result. It can return empty array, which is passed to self::getBackendUserAuthentication()->doesUserHaveAccess($page, 1), which fails inside TYPO3\CMS\Core\Authentication\BackendUserAuthentication::isInWebMount() because the value is empty array.

Stack trace:
TYPO3-Exception.html.zip

I will make the PR with the fix.