use the same check as in my_courses, fixes #557

lib/models/BlubberThread.php

@@ -459,7 +459,7 @@ class BlubberThread extends SimpleORMap implements PrivacyObject
                 if ($module = $tool->getStudipModule()) {
                     $last_visit = object_get_visit($this['context_id'], $module->getPluginId());
                     $nav = $module->getIconNavigation($this['context_id'], $last_visit, $GLOBALS['user']->id);
-                    if ($nav) {
+                    if (isset($nav) && $nav->isVisible(true)) {
                         $icons[] = $nav;
                     }
                     if ($module instanceof CoreSchedule) {