Co-authored-by: Loïc Sharma <737941+loic-sharma@users.noreply.github.com>

…eEvent` (flutter-team-archive#44344)
Previously, destruction of `Window` called `DestroyWindow`, which may
send `WM_KILLFOCUS` to the to-be-destroyed window. Because `Window`'s
destructor is called after `FlutterWindow`'s, the `FlutterWindow` vtable
was already destroyed at this point, and the subsequent call to the
virtual method `OnWindowStateEvent` would cause a crash. This PR
reintroduces the reverted changes for Windows lifecycle with a check
before calling the virtual method that the `FlutterWindow` object has
not yet been destructed.
flutter/flutter#131872
*If you had to change anything in the [flutter/tests] repo, include a
link to the migration guide as per the [breaking change policy].*
## Pre-launch Checklist
- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide] and the [C++,
Objective-C, Java style guides].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I added new tests to check the change I am making or feature I am
adding, or Hixie said the PR is test-exempt. See [testing the engine]
for instructions on writing and running engine tests.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [ ] I signed the [CLA].
- [x] All existing and new tests are passing.
If you need help, consider asking for advice on the #hackers-new channel
on [Discord].
<!-- Links -->
[Contributor Guide]:
https://github.com/flutter/flutter/wiki/Tree-hygiene#overview
[Tree Hygiene]: https://github.com/flutter/flutter/wiki/Tree-hygiene
[Flutter Style Guide]:
https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo
[C++, Objective-C, Java style guides]:
https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style
[testing the engine]:
https://github.com/flutter/flutter/wiki/Testing-the-engine
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[breaking change policy]:
https://github.com/flutter/flutter/wiki/Tree-hygiene#handling-breaking-changes
[Discord]: https://github.com/flutter/flutter/wiki/Chat
---------
Co-authored-by: Loïc Sharma <737941+loic-sharma@users.noreply.github.com>