TYPO3 Core

Cannot assign null to property TYPO3\CMS\Backend\Controller\Wizard\AddController::$returnEditConf of type string

in /var/www/html/typo3/sysext/backend/Classes/Controller/Wizard/AddController.php line 201
$parsedBody = $request->getParsedBody();
$queryParams = $request->getQueryParams();
// Init GPvars:
$this->P = $parsedBody['P'] ?? $queryParams['P'] ?? [];
$this->returnEditConf = $parsedBody['returnEditConf'] ?? $queryParams['returnEditConf'] ?? null;
// Get this record
$record = BackendUtility::getRecord($this->P['table'], $this->P['uid']);
// Set table:
$this->table = $this->P['params']['table'];

How to reproduce: use any AddRecord wizard, e.g. filemount in BEUser or styleguide -> elements_group -> group_db_1