[Windows] Fix crash when navigating pages

[Foda]

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:

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

[Foda]

Note: we don't just run this by default because when the transition is animated the page flashes blank.

[rmarinho]

Should we also clear/set to null _previousContent on Disconnect ?

[BaY1251]

There are similar issues on the Android platform #18331.

[PureWeen]

/backport to release/9.0.1xx

[github-actions]

Started backporting to release/9.0.1xx: https://github.com/dotnet/maui/actions/runs/11920307270

[AwsomeCode]

Hi @Foda
Can you please check this #26214