prevent error message when toolbar deleted by JS code; re #4836

git-svn-id: svn://develop.studip.de/studip/trunk@31818 c5b85792-5220-0410-ad23-ed5b6b8ad999
fixes #4830

git-svn-id: svn://develop.studip.de/studip/trunk@31819 c5b85792-5220-0410-ad23-ed5b6b8ad999