Vtiger 7 Shared calendar is not loading after Google Sync

So after fixing the issues in Google Calendar, for which I shall send a merge request, shared calendar has stopped working we get following error:

Notice: Undefined index: CURRENT_USER_MODEL in /var/www/vhosts/example.com/test/templates_c/v7/072d70ff812cf9c4727d578af221ce0d3cc1fb0a.file.SharedCalendarView.tpl.php on line 38

Notice: Trying to get property of non-object in /var/www/vhosts/example.com/test/templates_c/v7/072d70ff812cf9c4727d578af221ce0d3cc1fb0a.file.SharedCalendarView.tpl.php on line 38

Fatal error: Call to a member function get() on null in /var/www/vhosts/example.com/test/templates_c/v7/072d70ff812cf9c4727d578af221ce0d3cc1fb0a.file.SharedCalendarView.tpl.php on line 38