[Windows] Fix "PlatformView cannot be null here" exception during handler disconnect

src/Controls/src/Core/VisualElement/VisualElement.cs

@@ -1674,26 +1674,37 @@ private protected void SetPointerOver(bool value, bool callChangeVisualState = t
 		/// </summary>
 		protected internal virtual void ChangeVisualState()
 		{
-			if (!IsEnabled)
+			try
 			{
-				VisualStateManager.GoToState(this, VisualStateManager.CommonStates.Disabled);
-			}
-			else
-			{
-				bool isSelected = this.IsElementInSelectedState();
-				string targetState = isSelected ? VisualStateManager.CommonStates.Selected
-												: (IsPointerOver ? VisualStateManager.CommonStates.PointerOver : VisualStateManager.CommonStates.Normal);
+				if (!IsEnabled)
+				{
+					VisualStateManager.GoToState(this, VisualStateManager.CommonStates.Disabled);
+				}
+				else
+				{
+					bool isSelected = this.IsElementInSelectedState();
+					string targetState = isSelected ? VisualStateManager.CommonStates.Selected
+													: (IsPointerOver ? VisualStateManager.CommonStates.PointerOver : VisualStateManager.CommonStates.Normal);
 
-				VisualStateManager.GoToState(this, targetState);
-			}
+					VisualStateManager.GoToState(this, targetState);
+				}
 
-			if (IsEnabled)
+				if (IsEnabled)
+				{
+					// Focus needs to be handled independently; otherwise, if no actual Focus state is supplied
+					// in the control's visual states, the state can end up stuck in PointerOver after the pointer
+					// exits and the control still has focus.
+					VisualStateManager.GoToState(this,
+						IsFocused ? VisualStateManager.CommonStates.Focused : VisualStateManager.CommonStates.Unfocused);
+				}
+			}
+			catch (InvalidOperationException)
 			{
-				// Focus needs to be handled independently; otherwise, if no actual Focus state is supplied
-				// in the control's visual states, the state can end up stuck in PointerOver after the pointer
-				// exits and the control still has focus.
-				VisualStateManager.GoToState(this,
-					IsFocused ? VisualStateManager.CommonStates.Focused : VisualStateManager.CommonStates.Unfocused);
+				// Swallow "PlatformView cannot be null here" thrown when a visual state cascade fans out
+				// during handler disconnect (e.g. on Windows: navigating away from a focused control runs
+				// UpdateIsFocused(false) inside DisconnectHandler -> ChangeVisualState -> VSM Setter ->
+				// mapper -> strongly-typed PlatformView accessor). The handler/PlatformView has already
+				// been released, so there is nothing for the mapper to update. See dotnet/maui#27101.
 			}
 		}
 

src/Controls/tests/TestCases.HostApp/Issues/Issue27101.xaml

@@ -0,0 +1,58 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<local:TestContentPage xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
+				 xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+				 xmlns:local="clr-namespace:Maui.Controls.Sample.Issues"
+				 x:Class="Maui.Controls.Sample.Issues.Issue27101MainPage">
+	<local:TestContentPage.Resources>
+		<ResourceDictionary>
+			<Style x:Key="Issue27101ButtonStyle" TargetType="Button">
+				<Setter Property="TextColor" Value="White" />
+				<Setter Property="BackgroundColor" Value="Purple" />
+				<Setter Property="FontFamily" Value="OpenSansRegular"/>
+				<Setter Property="FontSize" Value="14"/>
+				<Setter Property="BorderWidth" Value="0"/>
+				<Setter Property="CornerRadius" Value="8"/>
+				<Setter Property="Padding" Value="14,10"/>
+				<Setter Property="MinimumHeightRequest" Value="44"/>
+				<Setter Property="MinimumWidthRequest" Value="44"/>
+				<Setter Property="VisualStateManager.VisualStateGroups">
+					<VisualStateGroupList>
+						<VisualStateGroup x:Name="CommonStates">
+							<VisualState x:Name="Normal" />
+							<VisualState Name="Focused" />
+							<VisualState x:Name="Disabled">
+								<VisualState.Setters>
+									<Setter Property="TextColor" Value="{AppThemeBinding Light=Black, Dark=LightGray}" />
+									<Setter Property="BackgroundColor" Value="{AppThemeBinding Light=LightGray, Dark=Gray}" />
+								</VisualState.Setters>
+							</VisualState>
+							<VisualState Name="PointerOver">
+								<VisualState.Setters>
+									<Setter Property="BackgroundColor" Value="Pink" />
+								</VisualState.Setters>
+							</VisualState>
+							<VisualState Name="Pressed">
+								<VisualState.Setters>
+									<Setter Property="BackgroundColor" Value="Red" />
+								</VisualState.Setters>
+							</VisualState>
+						</VisualStateGroup>
+					</VisualStateGroupList>
+				</Setter>
+			</Style>
+		</ResourceDictionary>
+	</local:TestContentPage.Resources>
+	<local:TestContentPage.Content>
+		<VerticalStackLayout
+			Padding="30,0"
+			Spacing="25">
+			<Button
+				x:Name="NavigateButton"
+				AutomationId="NavigateButton"
+				Text="Navigate to Second Page"
+				Clicked="OnNavigateButtonClicked"
+				HorizontalOptions="Fill"
+				Style="{StaticResource Issue27101ButtonStyle}"/>
+		</VerticalStackLayout>
+	</local:TestContentPage.Content>
+</local:TestContentPage>

src/Controls/tests/TestCases.HostApp/Issues/Issue27101.xaml.cs

@@ -0,0 +1,27 @@
+namespace Maui.Controls.Sample.Issues
+{
+	[Issue(IssueTracker.Github, 27101, "PlatformView cannot be null here Exception in Windows", PlatformAffected.UWP)]
+	public class Issue27101 : NavigationPage
+	{
+		public Issue27101() : base(new Issue27101MainPage())
+		{
+		}
+	}
+
+	public partial class Issue27101MainPage : TestContentPage
+	{
+		public Issue27101MainPage()
+		{
+			InitializeComponent();
+		}
+
+		protected override void Init()
+		{
+		}
+
+		void OnNavigateButtonClicked(object sender, EventArgs e)
+		{
+			Navigation.PushAsync(new Issue27101SecondPage());
+		}
+	}
+}

src/Controls/tests/TestCases.HostApp/Issues/Issue27101SecondPage.xaml

@@ -0,0 +1,58 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<ContentPage xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
+			 xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+			 xmlns:local="clr-namespace:Maui.Controls.Sample.Issues"
+			 x:Class="Maui.Controls.Sample.Issues.Issue27101SecondPage">
+	<ContentPage.Resources>
+		<ResourceDictionary>
+			<Style x:Key="Issue27101ButtonStyle" TargetType="Button">
+				<Setter Property="TextColor" Value="White" />
+				<Setter Property="BackgroundColor" Value="Purple" />
+				<Setter Property="FontFamily" Value="OpenSansRegular"/>
+				<Setter Property="FontSize" Value="14"/>
+				<Setter Property="BorderWidth" Value="0"/>
+				<Setter Property="CornerRadius" Value="8"/>
+				<Setter Property="Padding" Value="14,10"/>
+				<Setter Property="MinimumHeightRequest" Value="44"/>
+				<Setter Property="MinimumWidthRequest" Value="44"/>
+				<Setter Property="VisualStateManager.VisualStateGroups">
+					<VisualStateGroupList>
+						<VisualStateGroup x:Name="CommonStates">
+							<VisualState x:Name="Normal" />
+							<VisualState Name="Focused" />
+							<VisualState x:Name="Disabled">
+								<VisualState.Setters>
+									<Setter Property="TextColor" Value="{AppThemeBinding Light=Black, Dark=LightGray}" />
+									<Setter Property="BackgroundColor" Value="{AppThemeBinding Light=LightGray, Dark=Gray}" />
+								</VisualState.Setters>
+							</VisualState>
+							<VisualState Name="PointerOver">
+								<VisualState.Setters>
+									<Setter Property="BackgroundColor" Value="Pink" />
+								</VisualState.Setters>
+							</VisualState>
+							<VisualState Name="Pressed">
+								<VisualState.Setters>
+									<Setter Property="BackgroundColor" Value="Red" />
+								</VisualState.Setters>
+							</VisualState>
+						</VisualStateGroup>
+					</VisualStateGroupList>
+				</Setter>
+			</Style>
+		</ResourceDictionary>
+	</ContentPage.Resources>
+	<ContentPage.Content>
+		<VerticalStackLayout
+			Padding="30,0"
+			Spacing="25">
+			<Button
+				x:Name="NavigateBackButton"
+				AutomationId="NavigateBackButton"
+				Text="Navigate Back"
+				Clicked="OnNavigateBackButtonClicked"
+				HorizontalOptions="Fill"
+				Style="{StaticResource Issue27101ButtonStyle}"/>
+		</VerticalStackLayout>
+	</ContentPage.Content>
+</ContentPage>

src/Controls/tests/TestCases.HostApp/Issues/Issue27101SecondPage.xaml.cs

@@ -0,0 +1,15 @@
+namespace Maui.Controls.Sample.Issues
+{
+	public partial class Issue27101SecondPage : ContentPage
+	{
+		public Issue27101SecondPage()
+		{
+			InitializeComponent();
+		}
+
+		async void OnNavigateBackButtonClicked(object sender, EventArgs e)
+		{
+			await Navigation.PopAsync();
+		}
+	}
+}

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Issue27101.cs

@@ -0,0 +1,34 @@
+using NUnit.Framework;
+using UITest.Appium;
+using UITest.Core;
+
+namespace Microsoft.Maui.TestCases.Tests.Issues;
+
+public class Issue27101 : _IssuesUITest
+{
+	public Issue27101(TestDevice device)
+		: base(device)
+	{ }
+
+	public override string Issue => "PlatformView cannot be null here Exception in Windows";
+
+	[Test]
+	[Category(UITestCategories.Button)]
+	public void NoCrashWhenNavigatingBackFromPageWithFocusedButton()
+	{
+		// The crash reproduces only when a Button with VisualStates currently holds focus
+		// at the moment the page is navigated away from. Repeat the navigation a few times
+		// to give the focus race a chance to surface.
+		for (int i = 0; i < 10; i++)
+		{
+			App.WaitForElement("NavigateButton");
+			App.Tap("NavigateButton");
+
+			App.WaitForElement("NavigateBackButton");
+			App.Tap("NavigateBackButton");
+		}
+
+		// If we got here without an unhandled InvalidOperationException, the regression is fixed.
+		App.WaitForElement("NavigateButton");
+	}
+}