`NullReferenceExcetion` when binding `RadioButtons` using `WindowsToolkit.WinUI.Extensions`

### Describe the bug

```

```


It's only when the value is changed and is trying to be bound to code. So `OneWay` works.
Work around it like this:

```

```

Binding to `ComboBox` worked as expected with `TwoWay`.

### Steps to reproduce

```text
1. See above
```

### Expected behavior

Bind two-way

### Screenshots

_No response_

### Code Platform

- [ ] UWP
- [X] WinAppSDK / WinUI 3
- [ ] Web Assembly (WASM)
- [ ] Android
- [ ] iOS
- [ ] MacOS
- [ ] Linux / GTK

### Windows Build Number

- [ ] Windows 10 1809 (Build 17763)
- [ ] Windows 10 1903 (Build 18362)
- [ ] Windows 10 1909 (Build 18363)
- [ ] Windows 10 2004 (Build 19041)
- [ ] Windows 10 20H2 (Build 19042)
- [ ] Windows 10 21H1 (Build 19043)
- [ ] Windows 10 21H2 (Build 19044)
- [ ] Windows 10 22H2 (Build 19045)
- [X] Windows 11 21H2 (Build 22000)
- [ ] Other (specify)

### Other Windows Build number

22621.3296

### App minimum and target SDK version

- [ ] Windows 10, version 1809 (Build 17763)
- [ ] Windows 10, version 1903 (Build 18362)
- [ ] Windows 10, version 1909 (Build 18363)
- [ ] Windows 10, version 2004 (Build 19041)
- [ ] Windows 10, version 2104 (Build 20348)
- [X] Windows 11, version 22H2 (Build 22000)
- [ ] Other (specify)

### Other SDK version

_No response_

### Visual Studio Version

2022

### Visual Studio Build Number

_No response_

### Device form factor

_No response_

### Additional context

_No response_

### Help us help you

No, I'm unable to contribute a solution.

Inizia riproducendo l’esempio di RadioButtons dell’issue in WinAppSDK/WinUI 3 con il TwoWay binding e l’enum ItemsSource mostrati. Confrontalo con il ComboBox binding funzionante e il OneWay workaround; il lavoro è completato quando il TwoWay binding non genera più una NullReferenceException al cambio della selezione.

Scritto dal modello di indicizzazione a partire dal testo della issue.