Improve handling of missing grade items

Author: bwalkerl

classes/helper.php

@@ -229,6 +229,9 @@ public static function get_grade_context(\stdClass $record) {
         }
 
         $grade = new \grade_grade(['itemid' => $itemid], false);
+        if (empty($grade) || empty($grade->id)) {
+            return false;
+        }
         return $grade->get_context();
     }