disable courseware navigation for public access, fixes #1613

Closes #1613

Merge request studip/studip!2142

lib/modules/CoursewareModule.class.php

@@ -46,6 +46,10 @@ class CoursewareModule extends CorePlugin implements SystemPlugin, StudipModule
      */
     public function getTabNavigation($courseId)
     {
+        if ($GLOBALS['user']->id === 'nobody') {
+            return [];
+        }
+
         $navigation = new Navigation(
             _('Courseware'),
             URLHelper::getURL('dispatch.php/course/courseware/?cid='.$courseId)
@@ -80,6 +84,10 @@ class CoursewareModule extends CorePlugin implements SystemPlugin, StudipModule
      */
     public function getIconNavigation($courseId, $last_visit, $user_id)
     {
+        if ($user_id === 'nobody') {
+            return null;
+        }
+
         $statement = DBManager::get()->prepare("
                 SELECT COUNT(DISTINCT elem.id)
                 FROM `cw_structural_elements` AS elem