Redirection warning when creating personal flickr public repository

Encountered this while testing MDL-38500.

Replication steps:

1. Enable Flickr Public repository (Enable User/Course instances, the API does not need to be valid)
2. Login as a student
3. Go to My home ► My profile ► Repositories ► XXX ► Repositories
4. Add a new instance of Flickr Public named BOB

The following warning is shown.

You should really redirect before you start page output

line 728 of /lib/outputrenderers.php: call to debugging()

line 2540 of /lib/weblib.php: call to core_renderer->redirect_message()

line 193 of /repository/manage_instances.php: call to redirect()