[Windows] Fix crash when navigating pages (#25740)

### Description of Change

Fixes a long-standing crash when navigating pages:

1. Manually clear the `ContentPresenter.Content` property.

When navigating pages it was discovered that the `Content` property of
the `ContentPresenter` sometimes was not being detached from the parent
`ContentPresenter` (the `ContentPresenter` itself does detach from the
Page). A workaround for this was to manually clear the `Content`
property.

**The actual cause of this bug:** I suspect that something causing the
lifecycle tracking to be messed up. I'm still not sure what this is, but
for now this will help our customers more than nothing.

## Workaround

The following can be used as a workaround. Place it in the `Loaded`
event for your `AppShell`:

```csharp
private void AppShell_Loaded(object? sender, EventArgs e)
{
    Loaded -= AppShell_Loaded;

#if WINDOWS
    if (Handler != null && Handler.PlatformView is ShellView shellView &&
        shellView.Content is MauiNavigationView nv &&
        nv.Content is Microsoft.UI.Xaml.Controls.Frame frame)
    {
        frame.Navigating += (s, e) =>
        {
            if (frame.Content is Microsoft.UI.Xaml.Controls.Page page)
            {
                page.Unloaded += PageUnloaded;

                void PageUnloaded(object sender, Microsoft.UI.Xaml.RoutedEventArgs e)
                {
                    page.Unloaded -= PageUnloaded;
                    if (page.Content is Microsoft.UI.Xaml.Controls.ContentPresenter presenter)
                    {
                        presenter.Content = null;
                    }
                };
            }
        };
    }
#endif
}
```

### Issues Fixed

Fixes #22790 #18441 #22131

src/Controls/tests/DeviceTests/Elements/Shell/ShellTests.Windows.cs

@@ -541,6 +541,31 @@ await CreateHandlerAndAddToWindow<ShellHandler>(shell, async (handler) =>
 			});
 		}
 
+		[Fact(DisplayName = "Navigate back and forth doesn't crash")]
+		public async Task NavigateBackAndForthDoesntCrash()
+		{
+			SetupBuilder();
+
+			var shell = await CreateShellAsync(shell =>
+			{
+				shell.CurrentItem = new ContentPage();
+			});
+
+			await CreateHandlerAndAddToWindow<ShellHandler>(shell, async (handler) =>
+			{
+				var secondPage = new ContentPage();
+
+				for (int i = 0; i < 5; i++)
+				{
+					await shell.Navigation.PushAsync(secondPage, false);
+					await Task.Delay(100);
+					await shell.Navigation.PopToRootAsync(false);
+					await Task.Delay(100);
+				}
+				Assert.NotNull(shell.Handler);
+			});
+		}
+
 		[Fact(DisplayName = "Shell Toolbar With Only MenuBarItems Is Visible")]
 		public async Task ShellToolbarWithOnlyMenuBarItemsIsVisible()
 		{

src/Core/src/Platform/Windows/StackNavigationManager.cs

@@ -1,9 +1,6 @@
 using System;
 using System.Collections.Generic;
-using System.Linq;
-using System.Text;
 using System.Threading;
-using System.Threading.Tasks;
 using Microsoft.UI.Xaml;
 using Microsoft.UI.Xaml.Controls;
 using Microsoft.UI.Xaml.Media.Animation;
@@ -17,6 +14,7 @@ public class StackNavigationManager
 		IMauiContext _mauiContext;
 		Frame? _navigationFrame;
 		Action? _pendingNavigationFinished;
+		ContentPresenter? _previousContent;
 		bool _connected;
 
 		protected NavigationRootManager WindowManager => _mauiContext.GetNavigationRootManager();
@@ -58,6 +56,12 @@ public virtual void Disconnect(IStackNavigation navigationView, Frame navigation
 			FirePendingNavigationFinished();
 			_navigationFrame = null;
 			NavigationView = null;
+
+			if (_previousContent is not null)
+			{
+				_previousContent.Content = null;
+				_previousContent = null;
+			}
 		}
 
 		public virtual void NavigateTo(NavigationRequest args)
@@ -166,6 +170,14 @@ void OnNavigated(object sender, UI.Xaml.Navigation.NavigationEventArgs e)
 					VerticalAlignment = UI.Xaml.VerticalAlignment.Stretch
 				};
 
+				// There's some bug in our code, or the lifecycle of ContentControl, that is causing the content to
+				// never be removed from the parent...
+				if (_previousContent is not null)
+				{
+					_previousContent.Content = null;
+					_previousContent = null;
+				}
+
 				page.Content = presenter;
 			}
 			else
@@ -174,13 +186,15 @@ void OnNavigated(object sender, UI.Xaml.Navigation.NavigationEventArgs e)
 			}
 
 			// At this point if the Content isn't a ContentPresenter the user has replaced
-			// the conent so we just let them take control
+			// the content so we just let them take control
 			if (presenter == null || _currentPage == null)
 				return;
 
+			var platformPage = _currentPage.ToPlatform(MauiContext);
+
 			try
 			{
-				presenter.Content = _currentPage.ToPlatform(MauiContext);
+				presenter.Content = platformPage;
 			}
 			catch (Exception)
 			{
@@ -190,6 +204,8 @@ void OnNavigated(object sender, UI.Xaml.Navigation.NavigationEventArgs e)
 
 			_pendingNavigationFinished = () =>
 			{
+				_previousContent = presenter;
+
 				if (presenter?.Content is not FrameworkElement pc)
 				{
 					FireNavigationFinished();