PHP8: Fatal Error using Mailing Group input type

I have created a simple FP with an input of the type Mailing Group (multiple values allowed) and no action. On the try-out screen the following error is thrown.

TypeError: Illegal offset type in Civi\FormProcessor\EventListener\FormatSubscriber->onFormatDatabag() (line 89 of /buildkit/build/patch/web/sites/default/files/civicrm/ext/form-processor/Civi/FormProcessor/EventListener/FormatSubscriber.php)