Fix crash when closing the macos app from full screen

[bgoncal]

Summary

Strong reference was causing a crash when closing macos app from full screen.

Screenshots

Link to pull request in Documentation repository

Documentation: home-assistant/companion.home-assistant#

Any other notes

[codecov]

Codecov Report

Attention: 14 lines in your changes are missing coverage. Please review.

Comparison is base (498f279) 27.54% compared to head (4e3040c) 27.53%.
Report is 1 commits behind head on master.

Files	Patch %	Lines
Sources/App/Scenes/WebViewSceneDelegate.swift	0.00%	5 Missing ⚠️
Sources/App/Scenes/WindowScenesManager.swift	54.54%	5 Missing ⚠️
Sources/App/Scenes/WindowSizeObserver.swift	33.33%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2567      +/-   ##
==========================================
- Coverage   27.54%   27.53%   -0.01%     
==========================================
  Files         311      311              
  Lines       31681    31699      +18     
==========================================
+ Hits         8725     8727       +2     
- Misses      22956    22972      +16     

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