if no field group is selected, you don't get an error, and the extra field is saved in MYSQL, however the new extra field isn't listed (and can't be changed by the UI. then future extra fields can't be named the same thing (a sequential number is added to the end automatically), but you can't name the field the same thing and can't delete the 'root' item with that name.
i think you need need to have an error message that field group is required, OR remove the filter so that the field without a field group is still displayed in the list so that it can be modified.
