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

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")]
+	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");
+	}
+}

src/Core/src/Core/Extensions/InternalElementExtensions.cs

@@ -28,5 +28,15 @@ internal static bool IsConnectingHandler(this IElement element) =>
 		/// <returns></returns>
 		internal static bool IsReconnectingHandler(this IElement element) =>
 			(element.Handler as IElementHandlerStateExhibitor)?.State.HasFlag(ElementHandlerState.Reconnecting) ?? false;
+
+		/// <summary>
+		/// Indicates whether the element's handler is currently being disconnected.
+		/// While disconnecting, the platform view has already been released so property mappers
+		/// must not be invoked.
+		/// </summary>
+		/// <param name="element"></param>
+		/// <returns></returns>
+		internal static bool IsDisconnectingHandler(this IElement element) =>
+			(element.Handler as IElementHandlerStateExhibitor)?.State.HasFlag(ElementHandlerState.Disconnecting) ?? false;
 	}
 }

src/Core/src/Handlers/Element/ElementHandler.cs

@@ -103,6 +103,14 @@ public virtual void UpdateValue(string property)
 			if (VirtualView == null)
 				return;
 
+			// While the handler is being disconnected the platform view has already been released
+			// (see IElementHandler.DisconnectHandler below). Property changes that fan out from the
+			// virtual view during teardown (e.g. IsFocused -> ChangeVisualState -> VSM Setters)
+			// must not invoke mappers that touch the released platform view, otherwise strongly-typed
+			// PlatformView accessors throw "PlatformView cannot be null here".
+			if (this.IsDisconnectingHandler())
+				return;
+
 			_mapper?.UpdateProperty(this, VirtualView, property);
 		}
 
@@ -146,10 +154,22 @@ void IElementHandler.DisconnectHandler()
 				// DisconnectHandler
 				var oldPlatformView = PlatformView;
 				PlatformView = null;
-				DisconnectHandler(oldPlatformView);
+				try
+				{
+					// Mark the handler as disconnecting so property changes that fan out from the
+					// virtual view during platform teardown skip the mapper (see UpdateValue above).
+					_handlerState = ElementHandlerState.Disconnecting;
+					DisconnectHandler(oldPlatformView);
+				}
+				finally
+				{
+					_handlerState = ElementHandlerState.Disconnected;
+				}
+			}
+			else
+			{
+				_handlerState = ElementHandlerState.Disconnected;
 			}
-
-			_handlerState = ElementHandlerState.Disconnected;
 		}
 	}
 }

src/Core/src/Handlers/ElementHandlerState.cs

@@ -27,6 +27,15 @@ internal enum ElementHandlerState : byte
 		/// <summary>
 		/// The handler is connected to an element.
 		/// </summary>
-		Connected = 0x8
+		Connected = 0x8,
+		/// <summary>
+		/// The handler is in the process of disconnecting from its element.
+		/// While in this state the platform view has already been released
+		/// (and is no longer accessible via <see cref="IElementHandler.PlatformView"/>),
+		/// but the handler is still wired to the virtual view to allow platform
+		/// teardown to complete. Property mappers are short-circuited during this
+		/// window to avoid touching the released platform view.
+		/// </summary>
+		Disconnecting = 0x10
 	}
 }

src/Core/src/Handlers/InternalElementHandlerExtensions.cs

@@ -28,5 +28,15 @@ internal static bool IsConnectingHandler(this IElementHandler handler) =>
 		/// <returns></returns>
 		internal static bool IsReconnectingHandler(this IElementHandler handler) =>
 			(handler as IElementHandlerStateExhibitor)?.State.HasFlag(ElementHandlerState.Reconnecting) ?? false;
+
+		/// <summary>
+		/// Indicates whether the handler is currently being disconnected from its element.
+		/// While disconnecting, the platform view has already been released so property mappers
+		/// must not be invoked.
+		/// </summary>
+		/// <param name="handler"></param>
+		/// <returns></returns>
+		internal static bool IsDisconnectingHandler(this IElementHandler handler) =>
+			(handler as IElementHandlerStateExhibitor)?.State.HasFlag(ElementHandlerState.Disconnecting) ?? false;
 	}
 }

src/Core/tests/UnitTests/AbstractViewHandlerTests.cs

@@ -42,6 +42,39 @@ public void ConnectAndDisconnectFireAppropriateNumberOfTimes()
 			Assert.Equal(2, handlerStub.DisconnectHandlerCount);
 		}
 
+		[Fact]
+		public void UpdateValueIsSkippedWhileHandlerIsDisconnecting()
+		{
+			// Regression test for https://github.com/dotnet/maui/issues/27101
+			// While the handler is being disconnected the platform view has already been released.
+			// Property changes that fan out from the virtual view during teardown (e.g. IsFocused
+			// -> ChangeVisualState -> VSM Setters) must not invoke mappers, otherwise the strongly
+			// typed PlatformView accessor throws "PlatformView cannot be null here".
+
+			DisconnectingTrackingHandlerStub handlerStub = null;
+			handlerStub = new DisconnectingTrackingHandlerStub(() =>
+			{
+				// Simulate a property change that fans into the mapper while the platform handler
+				// is still tearing down (this mirrors what UpdateIsFocused(false) triggers on Windows).
+				handlerStub.UpdateValue(nameof(IView.Background));
+			});
+
+			handlerStub.SetVirtualView(new Maui.Controls.Button());
+
+			// Sanity check: outside of disconnect, UpdateValue routes through the mapper.
+			handlerStub.MapBackgroundCallCount = 0;
+			handlerStub.UpdateValue(nameof(IView.Background));
+			Assert.Equal(1, handlerStub.MapBackgroundCallCount);
+
+			// Reset so we can detect any mapper invocations that happen during DisconnectHandler.
+			handlerStub.MapBackgroundCallCount = 0;
+
+			(handlerStub as IViewHandler).DisconnectHandler();
+
+			// The mapper-routed UpdateValue inside DisconnectHandler must have been a no-op.
+			Assert.Equal(0, handlerStub.MapBackgroundCallCount);
+		}
+
 		[Fact]
 		public void GetRequiredServiceThrowsOnNoContext()
 		{

src/Core/tests/UnitTests/TestClasses/DisconnectingTrackingHandlerStub.cs

@@ -0,0 +1,43 @@
+using System;
+using Microsoft.Maui.Handlers;
+
+namespace Microsoft.Maui.UnitTests
+{
+	// Helper used by the regression test for https://github.com/dotnet/maui/issues/27101.
+	// During DisconnectHandler we trigger an UpdateValue call to mimic the cascade caused on
+	// Windows by UpdateIsFocused(false) -> ChangeVisualState -> VSM Setters -> mapper. We then
+	// assert the mapper was NOT invoked while the handler was tearing down.
+	public class DisconnectingTrackingHandlerStub : ViewHandler<Maui.Controls.Button, object>
+	{
+		readonly Action _onDisconnect;
+
+		public int MapBackgroundCallCount { get; set; }
+
+		public DisconnectingTrackingHandlerStub(Action onDisconnect)
+			: base(BuildMapper())
+		{
+			_onDisconnect = onDisconnect;
+		}
+
+		static IPropertyMapper<IView, IViewHandler> BuildMapper()
+		{
+			var mapper = new PropertyMapper<IView, IViewHandler>();
+			mapper.Add(nameof(IView.Background), (handler, view) =>
+			{
+				if (handler is DisconnectingTrackingHandlerStub stub)
+				{
+					stub.MapBackgroundCallCount++;
+				}
+			});
+			return mapper;
+		}
+
+		protected override object CreatePlatformView() => new object();
+
+		protected override void DisconnectHandler(object platformView)
+		{
+			base.DisconnectHandler(platformView);
+			_onDisconnect?.Invoke();
+		}
+	}
+}