fix "UnexpectedValueException: Invalid context"

AvailSchedule.class.php

@@ -26,7 +26,9 @@ class AvailSchedule extends StudipPlugin implements SystemPlugin
     {
         parent::__construct();
 
-        $sem_type = Context::getArtNum();
+        if (Context::getId()) {
+            $sem_type = Context::getArtNum();
+        }
 
         if (isset($sem_type) && !$GLOBALS['SEM_CLASS'][$GLOBALS['SEM_TYPE'][$sem_type]['class']]['studygroup_mode'] &&
             Navigation::hasItem('/course/members') && $GLOBALS['perm']->have_studip_perm('tutor', Context::getId())) {