Argument 1 passed to TYPO3\CMS\Backend\View\ BackendLayout\BackendLayoutCollection:: add() must be an instance of TYPO3\CMS\Backend\View\ BackendLayout\BackendLayout, null given,

Der Fehler

TypeError
Argument 1 passed to TYPO3\CMS\Backend\View\BackendLayout\BackendLayoutCollection::add() must be an instance of TYPO3\CMS\Backend\View\BackendLayout\BackendLayout, null given, called in /var/www/html/typo3conf/ext/fluidpages/Classes/Backend/BackendLayoutDataProvider.php on line 92

lässt sich folgendermaßen umgehen:
In der Datei typo3conf/ext/fluidpages/Classes/Backend/BackendLayoutDataProvider.php muss die Zeile 92 in folgendes umgewandelt werden:

$backendLayout = $this->getBackendLayout('fluidpages', $dataProviderContext->getPageId());
if ($backendLayout) $backendLayoutCollection->add($backendLayout);