MDL-43199 assign: fix js error when expanding comments for non editing teachers in assign grading table

mod/assign/module.js

@@ -150,9 +150,11 @@ M.mod_assign.init_grading_options = function(Y) {
             });
         }
         var showonlyactiveenrolelement = Y.one('#id_showonlyactiveenrol');
-        showonlyactiveenrolelement.on('change', function(e) {
+        if (showonlyactiveenrolelement) {
+            showonlyactiveenrolelement.on('change', function(e) {
             Y.one('form.gradingoptionsform').submit();
-        });
+            });
+        }
     });
 };