1. Make sure debugging is on
2. Enable async course backups
3. Backup a course and run cron
4. Download the resulting backup file
5. Go to Site admin > Course > Restore
6. Upload the backup and proceed to restore the course into a new course
7. When you are asked to wait (when the async restore in being processed), you'll see a php notice:

   Notice: Trying to get property 'id' of non-object

The notice comes from this line.

In this case, $context is system, and $course is null.

So, the 'return to course' button won't work (it doesn't).