Fix for navigation breaking when students have no view rights on one of the level one categories

classes/local/helpers/webservicehelper.php

@@ -74,6 +74,12 @@ class webservicehelper {
         // Initialize parent if needed.
         if ($parent == null) {
             $parent = \core_course_category::user_top();
+
+            if ($parent == null) {
+                // If the parent is still null, we do not have a valid user top.
+                return false;
+            }
+
             if (has_capability($capability, $parent->get_context())) {
                 $list[] = $parent;
             }