[SebastianKra]

That's a bad excuse. There are plenty of widgets that need to deal with illegal states in an auto-saving form. For example, every text input that expects a numeric value needs to allow an empty string.

If you don't want to deal with validation logic in your app, you could just disable the last checkbox.

[sureIy]

I like your last point.

Also in practice this state does not happen in “apps”. Before the user reaches the screen, you must have set a default. If you have default, then “deselect all” will eventually revert to that default.