NullReferenceException in CommunityToolkit.WinUI.Behaviors StackedNotificationsBehavior.ShowNext

### Describe the bug

I have not reproduced this locally, but I'm seeing this NullReferenceException occasionally from user crash reports:

```
CommunityToolkit.WinUI.Behaviors
StackedNotificationsBehavior.ShowNext ()
CommunityToolkit.WinUI.Behaviors
StackedNotificationsBehavior.Show (Notification)
CommunityToolkit.WinUI.Behaviors
StackedNotificationsBehavior.Show (String, Int32, String)
```

### Steps to reproduce

```text
Unknown, unfortunately. My guess is the StackedNotificationsBehavior was used after its hosting ContentDialog had been closed.
```

### Expected behavior

I'd expect this component to not throw a NullReferenceException, even if used after being uninitialized.

### Screenshots

_No response_

### Code Platform

- [X] UWP
- [ ] 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)
- [X] Windows 10 22H2 (Build 19045)
- [ ] Windows 11 21H2 (Build 22000)
- [ ] Other (specify)

### Other Windows Build number

_No response_

### App minimum and target SDK version

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

### Other SDK version

22621

### Visual Studio Version

2022

### Visual Studio Build Number

17.10.1

### Device form factor

Desktop

### Additional context

_No response_

### Help us help you

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