MaterialRadioButtonGroup allows multiple selects and unable to deselect first option.

## 🐛 Bug Report

Using MaterialRadioButtonGroup

I am able to select more than one option.
I am unable to deselect the first option but can deselect other options. The first option is always selected.

### Expected behavior
Can only select one option, automatically other options are deselected.

### Reproduction steps

Xaml

ViewModel
public List items { get => this.myItems.Split(',').ToList(); }

this.myItems is a string and is currently set to:

"663703a2-7de8-4f32-81a4-9494ca6daa10,773703a2-7de8-4f32-81a4-9494ca6daa10"

### Configuration

**Version:** 1.5.8

**Platform:**
- [x] :iphone: iOS
- [ ] :robot: Android
- [ ] :checkered_flag: WPF
- [ ] :earth_americas: UWP
- [ ] :apple: MacOS
- [ ] :tv: tvOS
- [ ] :monkey: Xamarin.Forms