MDL-39487 mod/assign: fixed ID number of grade item changing

mod/assign/locallib.php

@@ -2350,7 +2350,7 @@ class assign {
             return false;
         }
         $assign = clone $this->get_instance();
-        $assign->cmidnumber = $this->get_course_module()->id;
+        $assign->cmidnumber = $this->get_course_module()->idnumber;
 
         return assign_grade_item_update($assign, $gradebookgrade);
     }