Improve custom URL scheme handler robustness

[UdaraJay]

Fix for wry_web_view.rs/get_custom_task_uuid trying to access stale WryWebView.

thread 'tokio-runtime-worker' panicked at .../wry/src/wkwebview/class/wry_web_view.rs:152:10:
tried to access uninitialized instance variable

This happens sometimes when closing a WebviewWindow or calling win.close(). It causes a runtime panic and crashes the application when it occurs. Fixed by adding runtime checks to make sure webview_id and task_uuid are valid before they are used.

Potentially related: #1425.

[github-actions]

Package Changes Through d0faa12

There are 1 changes which include wry with minor

Planned Package Versions

The following package releases are the planned based on the context of changes in this pull request.

package	current	next
wry	0.48.1	0.49.0

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[zakhenry]

I can confirm this patch fixed a segfault in macos for me too. For me this occurred during shutdown of the application after closing windows and invoking app.run_iteration(...)

[FabianLars]

This should be fairly safe to merge as the only logic changes are the upfront validation check (that i don't think has that much of an impact but as long as we're not hitting issues with the frequency of the mutex locks we should keep it), and the removal of the unwrap which i'm always a fan of.

The only thing i'd like to change is to bring back the removed comments.