Fix display of ActionSheet in modal view on Windows

[japarson]

Description of Change

When we call DisplayActionSheet from a modal dialog, we encounter an issue with the MauiContext at the following line:

maui/src/Controls/src/Core/Platform/AlertManager/AlertManager.Windows.cs

Line 212 in d03a2c6

var current = sender.ToPlatform(VirtualView.RequireMauiContext());

In this situation, VirtualView.RequireMauiContext() will return the MauiContext for the Window. This is problematic as it causes some strange behavior when converting the view to a Handler. Here, we can see that the view's Handler will be different from the Window's Handler`.

maui/src/Core/src/Platform/ElementExtensions.cs

Lines 67 to 68 in d03a2c6

	if (handler?.MauiContext != null && handler.MauiContext != context)
	handler = null;

As a result, we end up getting a new Handler from the Windows MauiContext and setting it here:

maui/src/Core/src/Platform/ElementExtensions.cs

Line 96 in d03a2c6

view.Handler = handler;

When we set the new Handler, the modal dialog actually ends up disappearing, while the window itself is still in a modal state, rendering it unusable:

Furthermore, we end up getting a null pageParent which leads us to attempt showing the ActionSheet from the current view:

maui/src/Controls/src/Core/Platform/AlertManager/AlertManager.Windows.cs

Lines 219 to 220 in d03a2c6

	if (current != null && current is FrameworkElement mainPage)
	actionSheet.ShowAt(current);

However, when we execute this here we end up hitting an exception related to a null XamlRoot:

To make matters worse, when we catch the exception we attempt to use UI.Xaml.Window.Current to show the ActionSheet which is also null.

maui/src/Controls/src/Core/Platform/AlertManager/AlertManager.Windows.cs

Lines 227 to 228 in d03a2c6

	if (UI.Xaml.Window.Current != null && UI.Xaml.Window.Current.Content is FrameworkElement mainPage)
	actionSheet.ShowAt(mainPage);

We can avoid this whole debacle by using sender.RequireMauiContext() instead.

Demo

Issues Fixed

Fixes #14829