[BUGFIX] Fatal error: Invalid argument passed to static method set_usergroup()

Fixes: #63515
Change-Id: I6d1719b5cd5725890a34b7d7f6d8f7bfcc62d91f
Reviewed-on: http://review.typo3.org/35341
Reviewed-by: Xavier Perseguers <xavier@typo3.org>
Tested-by: Xavier Perseguers <xavier@typo3.org>