mindmap 0.0.24 throws an php error

I have 2 instances of nextcloud running nextcloud:

  Working opensuse 15.1 php 7.2.5, nextcloud 18.0.10.2

non woking opensuse 15.2 php 7.4.6, nextcloud 20.0.3.2

Every invocation of the mindmap module throws the following error:
0. /srv/www/htdocs/nextcloud/apps/whiteboard/lib/Db/User.php line 39
OC\Log\ErrorHandler::onError(2, "Use of undefine ... )", "/srv/www/htdocs ... p", 39, [])

1. /srv/www/htdocs/nextcloud/lib/public/AppFramework/Db/Entity.php line 68
   OCA\whiteboard\Db\User->__construct()
2. /srv/www/htdocs/nextcloud/lib/public/AppFramework/Db/QBMapper.php line 307
   OCP\AppFramework\Db\Entity::fromRow({id: "51",app_id ... "})
3. /srv/www/htdocs/nextcloud/lib/public/AppFramework/Db/QBMapper.php line 325
   OCP\AppFramework\Db\QBMapper->mapRowToEntity({id: "51",app_id ... "})
4. /srv/www/htdocs/nextcloud/apps/whiteboard/lib/Db/UserMapper.php line 65
   OCP\AppFramework\Db\QBMapper->findEntities(OC\DB\QueryBuilder\QueryBuilder {})
5. /srv/www/htdocs/nextcloud/apps/whiteboard/lib/Service/CollaborationEngine.php line 92
   OCA\whiteboard\Db\UserMapper->findAll("whiteboard", 15236)
6. /srv/www/htdocs/nextcloud/apps/whiteboard/lib/Controller/CollaborationController.php line 84
   OCA\whiteboard\Service\CollaborationEngine->removeUser()
7. /srv/www/htdocs/nextcloud/lib/private/AppFramework/Http/Dispatcher.php line 169
   OCA\whiteboard\Controller\CollaborationController->removeUser()
8. /srv/www/htdocs/nextcloud/lib/private/AppFramework/Http/Dispatcher.php line 100
   OC\AppFramework\Http\Dispatcher->executeController(OCA\whiteboard\C ... }}, "removeUser")
9. /srv/www/htdocs/nextcloud/lib/private/AppFramework/App.php line 152
   OC\AppFramework\Http\Dispatcher->dispatch(OCA\whiteboard\C ... }}, "removeUser")
10. /srv/www/htdocs/nextcloud/lib/private/Route/Router.php line 308
    OC\AppFramework\App::main("OCA\whiteboard ... r", "removeUser", OC\AppFramework\ ... {}, {action: null,_r ... "})
11. /srv/www/htdocs/nextcloud/lib/base.php line 1008
    OC\Route\Router->match("/apps/whiteboar ... r")
12. /srv/www/htdocs/nextcloud/index.php line 37
    OC::handleRequest()

As I have to upgrade the working opensuse to 15.2 (15.1 has EoL at end of January), I'm afraid, that updating the included php version will lead to the same errors. Is there a chance for a new version of mindmap or is there another way to keep mindmap working?

Yours
Hermann-Josef Beckers

Start at apps/whiteboard/lib/Db/User.php line 39 and reproduce the error when invoking the mindmap module on PHP 7.4.6 with Nextcloud 20.0.3.2. Compare the working PHP 7.2.5 environment and follow the UserMapper and CollaborationEngine calls shown in the trace; done means the invocation no longer raises the undefined-value error.