WinUI 3: ThemeListener crashes

[ocalvo]

Describe the bug

Creating ThemeListener crashes

• Is this bug a regression in the toolkit? If so, what toolkit version did you last see it work:

Steps to Reproduce

• Can this be reproduced in the Sample App? Yes

Steps to reproduce the behavior:

1. Create a new WinUI3 Desktop app using version latest internal version on WinUI3
2. Add a reference to WCT preview 3:

3. Write this code in App.xaml.cs:

  protected override void OnLaunched(Microsoft.UI.Xaml.LaunchActivatedEventArgs args)
  {
     var t = new Microsoft.Toolkit.Uwp.UI.Helpers.ThemeListener();

4. F5

Expected behavior

App does not crash.

Screenshots

If applicable, add screenshots to help explain your problem.

Environment

NuGet Package(s): 
8.0.0-preview3

Package Version(s): 
WinUI3 latest internal version

Windows 10 Build Number:
- [ ] Fall Creators Update (16299)
- [ ] April 2018 Update (17134)
- [ ] October 2018 Update (17763)
- [ ] May 2019 Update (18362)
- [x] May 2020 Update (19041)
- [ ] Insider Build (build number: )

App min and target version:
- [ ] Fall Creators Update (16299)
- [ ] April 2018 Update (17134)
- [ ] October 2018 Update (17763)
- [x] May 2019 Update (18362)
- [ ] May 2020 Update (19041)
- [ ] Insider Build (xxxxx)

Device form factor:
- [x] Desktop
- [ ] Xbox
- [ ] Surface Hub
- [ ] IoT

Visual Studio 
- [ ] 2017 (version: )
- [ ] 2019 (version: ) 
- [x] 2019 Preview (version: 2)

Additional context

Add any other context about the problem here.

Hello ocalvo, thank you for opening an issue with us!

I have automatically added a "needs triage" label to help get things started. Our team will analyze and investigate the issue, and escalate it to the relevant team if possible. Other community members may also look into the issue and provide feedback 🙌

[Marv51]

Probably related to this entry in the WinUI 3 release note:
"The UISettings.ColorValuesChanged Event and AccessibilitySettings.HighContrastChanged Event are no longer supported in desktop apps. This may cause issues if you are using it to detect changes in Windows themes."