Verified on VS 17.10.0 Preview 4.0(8.0.20). Can repro this issue on iOS 17.2.
WebViewCrashMidLoad.zip

I've added the repro to our sandbox project here

https://github.com/dotnet/maui/tree/fix_21948

The bug here is that our code that searches for the keywindow doesn't work because mopups is using it's own UIWindow

So when our life cycle code tries to search for the keywindow it doesn't return anything.

Some possible thoughts on fixes

1. in a single window scenario (no SceneDelegate) just locate the one UIWindow
2. instead of tying lifecycle events to the delegate events can we inherit from UIWindow and tie activate/deactive to events inside UIWindow instead so that it's a little closer to the source?

we should probably do one regardless if two is a better solution for this scenario

MopupsCrash.zip

Workaround

@PureWeen
Observed the same issue on iOS only with mopups, tried to use your work around. This is not working when UIWindow from binding library is the key window, as IWindow.Activated is not triggered when base.OnActivated(application) is called in AppDelegate(after minimize and try to bring app to foreground). As the window created by maui is not activated in previous scenario, we will see an exception in OnResignActivation that the window is already deactivated.

@PureWeen

I have added below in OnActivated, this is helping as a work around for scenario mentioned in previous comment:

public override void OnActivated(UIApplication application)
{
            base.OnActivated(application);

            if (Application.Windows.FirstOrDefault() is App.WindowFix windowFix && !windowFix.IsActivated)
            {                
                (windowFix as IWindow).Activated();                
            }
}