Undefined index: NEWLY_REGISTERED when removing 'newly user' from default group

I was testing this with Raimon because some strange things happened, this is causing it:

When removing a "newly registered user" from his default group, let's say "Global Moderators" his default group is removed, but no other default group is being set.

You need debug enabled to see the error:

[phpBB Debug] PHP Notice: in file /includes/functions_user.php on line 2904: Undefined index: NEWLY_REGISTERED

The message "Users removed from group and new defaults set successfully." does appear and the user is removed from that group, but he keeps his color & rank.