Flutter group_button package throwing errors when inserting parameters and single-select

Question

New user to flutter (25 years of procedural coding, going a bit insane)

1. When using the simple logic on pub.dev it works ok: GroupButton( isRadio: false, buttons: ["Male", "Female", "Other"], ),

But as soon as I insert any of the options, it throws an error (the named parameter isn't defined): GroupButton( isRadio: false, spacing: 10, buttons: ["Male", "Female", "Other"], ),

2. What's the simplest way to put a border around the selected object and reset the others?

Help much appreciated (including a better method, etc.),

David