Comment 4 for bug 1558635

Looking at the code there is no reason that the interface should not be set. The only reason would because the interface did not get created. If that is the case the create with the form should raise an exception to tell the user.

Please check the following so I can see how this can be handled better:

Check that the MAC address you provided was unique. If it was not unique it should raise an exception.

Also check for any other logs that might be related to see this issue, like something in maas.log.