Exception thrown in FE when not logged in at Backend.

Core: Exception handler (WEB): Uncaught TYPO3 Exception: Call to a member function simplelog() on null | Error thrown in file /server/data/www/apache2.2/releases/97/web/typo3conf/ext/pb_social/Classes/Service/CacheService.php in line 151. Requested URL: XXX

There will be thrown an exception, when you are not logged in at Backend and try to load a page with pb_social Plugin in Frontend while an exception occured on loading posts from cache.