Moving a topic to subforum leads to exception

[sgrossberndt]

When moving a topic from a forum to a subforum of this forum an exception is thrown:

Call to a member function getRootline() on null
Error thrown in file typo3_forum/Classes/Domain/Model/Forum/Topic.php in line 392
return $subject->{$getterMethodName}();

Looking at the topic in backend reveals a shadow topic has been created and assigned successfully but the original topic no longer is assigned to any forum.

Manually assigning the forum in backend does not help either as then another exception is thrown on every request to the frontend:

#1476107295: PHP Warning: class_parents(): Class Topic does not exist and could not be loaded in /typo3_src-8.7.26/typo3/sysext/extbase/Classes/Persistence/Generic/Mapper/DataMapper.php line 282