reindex-exercises is broken

[anandology]

Describe the bug

Reindex Exercises command on The Joy of Programming course failed with the following traceback.

Traceback (most recent call last):
  File "/home/fossunited/frappe-bench/apps/frappe/frappe/app.py", line 66, in application
    response = frappe.api.handle()
  File "/home/fossunited/frappe-bench/apps/frappe/frappe/api.py", line 54, in handle
    return frappe.handler.handle()
  File "/home/fossunited/frappe-bench/apps/frappe/frappe/handler.py", line 31, in handle
    data = execute_cmd(cmd)
  File "/home/fossunited/frappe-bench/apps/frappe/frappe/handler.py", line 67, in execute_cmd
    return frappe.call(method, **frappe.form_dict)
  File "/home/fossunited/frappe-bench/apps/frappe/frappe/__init__.py", line 1205, in call
    return fn(*args, **newargs)
  File "/home/fossunited/frappe-bench/apps/community/community/lms/doctype/lms_course/lms_course.py", line 366, in reindex_exercises
    course.reindex_exercises()
  File "/home/fossunited/frappe-bench/apps/community/community/lms/doctype/lms_course/lms_course.py", line 230, in reindex_exercises
    c.save()
TypeError: 'NoneType' object is not callable

To Reproduce
Steps to reproduce the behavior:

1. Go to https://mon.school/app/lms-course/the-joy-of-programming
2. Click on Reindex Exercises action
3. See error message and traceback in the console log

Expected behavior
It should reindex all the exercises.