Steps to reproduce:

1. Log in as teacher
2. Create forum with 'Subscription Mode' set to 'Auto subscription'
3. Save and you will see following error

   Debug info: SELECT id,course FROM {course_modules} WHERE id IS NULL

   [array (

   )]

   Error code: invalidrecord

   Stack trace:

   line 1451 of /lib/dml/moodle_database.php: dml_missing_record_exception thrown

   line 1427 of /lib/dml/moodle_database.php: call to moodle_database->get_record_select()

   line 7074 of /lib/accesslib.php: call to moodle_database->get_record()

   line 4832 of /mod/forum/lib.php: call to context_module::instance()

   line 132 of /mod/forum/lib.php: call to forum_subscribe()

   line 113 of /course/modlib.php: call to forum_add_instance()

   line 268 of /course/modedit.php: call to add_moduleinfo()

   Alternatively run: MDLQA-6700 Step 5