Deprecation warning bricks ajax callback in Maintenance Module

[helmutstrasser]

If one flushes the cache in the backend module "Maintenance", the ajax callback answers with <br /> <b>Deprecated</b>: JosefGlatz\BeuserFastswitch\Controller\BackendController::userLookupAction(): Implicitly marking parameter $response as nullable is deprecated, the explicit nullable type must be used instead in <b>/var/www/html/vendor/josefglatz/beuser-fastswitch/Classes/Controller/BackendController.php</b> on line <b>52</b><br /> {"success":true,"status":[{"severity":0,"title":"Caches cleared","message":"Successfully cleared all caches and all available opcode caches.","storeInSession":false}]}

The second parameter $response can be removed anyway, because it is not used in the method userLookupAction().