cdodge/fix-crash-on-course-not-found: add null guard

Add logging message

lms/djangoapps/api_manager/users/views.py

@@ -613,6 +613,9 @@ class UsersCoursesList(SecureAPIView):
         response_data = []
         for enrollment in enrollments:
             descriptor = store.get_course(enrollment.course_id)
+            # NOTE: It is possible that a course has been hard deleted from the courseware
+            # database, but the enrollment row in the SQL database still exists
+            if descriptor:
                 course_data = {
                     "id": enrollment.course_id,
                     "uri": '{}/{}'.format(base_uri, enrollment.course_id),
@@ -620,6 +623,9 @@ class UsersCoursesList(SecureAPIView):
                     "name": descriptor.display_name
                 }
                 response_data.append(course_data)
+            else:
+                log.warning("User {0} enrolled in course_id {1}, but course could not be found.".format(user_id, enrollment.course_id))
+
         return Response(response_data, status=status.HTTP_200_OK)