Editing a group router forces you to define a new one

![screenshot 2014-05-27 11 12 16](https://cloud.githubusercontent.com/assets/1228977/3089489/570697b6-e57f-11e3-88da-dad60c0d60ae.png)

Checking the "Delete" box on the new one lets you continue, but that's not very optimal :grin: 
Having an invalid default choice forces one to make a choice instead of accidentally getting the (valid) default. I'd rather get a validation failure if I forget to choose a group than silently get a group I didn't explicitly choose, especially since the first group in the list is unlikely to be the desired selection most of the time.