fix: invalid cast exception with nullable appstate

[RLittlesII]

What kind of change does this PR introduce?

Fixes an InvalidCastException when observing app state change.

What is the current behavior?

fixes: #2465

What is the new behavior?

The invalid cast exception is fixed.

What might this PR break?

SuspensionHost extensions

Please check if the PR fulfills these requirements

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Other information:

[codecov]

Codecov Report

Merging #2478 into main will increase coverage by 0.63%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##             main    #2478      +/-   ##
==========================================
+ Coverage   51.86%   52.49%   +0.63%     
==========================================
  Files         114      114              
  Lines        4525     4522       -3     
  Branches      792      790       -2     
==========================================
+ Hits         2347     2374      +27     
+ Misses       1950     1916      -34     
- Partials      228      232       +4     

Impacted Files	Coverage Δ
.../ReactiveUI/Suspension/SuspensionHostExtensions.cs	73.07% <80.00%> (+73.07%)	⬆️
src/ReactiveUI.Winforms/RoutedViewHost.cs	82.19% <0.00%> (-2.74%)	⬇️
src/ReactiveUI/Suspension/SuspensionHost.cs	66.66% <0.00%> (+16.66%)	⬆️
src/ReactiveUI/Mixins/ObservableLoggingMixin.cs	19.04% <0.00%> (+19.04%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 2a6cd06...f5f450f. Read the comment docs.

[github-actions]

This pull request has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.