Use case

There is no validator property on DropdownMenu.
It is tricky to show error messages on DropdownMenu because of not knowing when the validation is being ran on the parent form.

Proposal

When using DropdownMenu, it'd be great to have a validator property, as we already do for FormField:
https://api.flutter.dev/flutter/widgets/FormField/validator.html