Advanced grading page doesn't display how guide score is converted to the grade for activities

Following message doesn't get displayed on the advanced grading page when required.

'WARNING: Your marking guide has a maximum grade of <b>{$a->maxscore} points</b> but the maximum grade set in your activity is {$a->modulegrade} The maximum score set in your marking guide will be scaled to the maximum grade in the module.<br />
Intermediate scores will be converted respectively and rounded to the nearest available grade.'

Steps to replicate: 

1.  Create a course 
2. Create a forum activity with Grade set to Point and Maximum grade = 50
3. Set Grading method to Marking guide
4. On advanced grading screen Define new grading from from scratch and create a couple of criterions with total max Score more than 50.
5. Save  marking guide and get it ready.
6. On advanced grading screen:

expected: Warning message displayed

actual: no warning message displayed 

This happens because for some reason forum activity  has "grade_forum" field in "forum" table, but rest of the activities e.g.  "lesson" or "assign" have it as "grade".

This breaks logic when we expect "grade" field to be available as part of "moduleinstance" object received here https://github.com/moodle/moodle/blob/master/grade/grading/form/guide/lib.php#L356

and used here https://github.com/moodle/moodle/blob/master/grade/grading/form/guide/renderer.php#L774