Android: ensure fragment has not been disposed while updating left ba…

…r button

Fixes #14801

src/Controls/src/Core/Compatibility/Handlers/Shell/Android/ShellToolbarTracker.cs

@@ -376,8 +376,17 @@ protected virtual async void UpdateLeftBarButtonItem(Context context, AToolbar t
 					ToolbarNavigationClickListener = this,
 				};
 
+				// TODO: Obsolete and Remove `UpdateDrawerArrowFromFlyoutIcon`
+				// Its original purpose was to set the icon from the FlyoutIcon which is now handled by GetFlyoutIcon below.
+				// See: https://github.com/xamarin/Xamarin.Forms/pull/6762
 				await UpdateDrawerArrowFromFlyoutIcon(context, _drawerToggle);
 
+				// Fragment might have been disposed while we were awaiting
+				if (_disposed)
+				{
+					return;
+				}
+
 				_drawerToggle.DrawerSlideAnimationEnabled = false;
 				drawerLayout.AddDrawerListener(_drawerToggle);
 			}
@@ -395,17 +404,27 @@ protected virtual async void UpdateLeftBarButtonItem(Context context, AToolbar t
 			var tintColor = Colors.White;
 			if (TintColor != null)
 				tintColor = TintColor;
-
+			
 			if (image != null)
 			{
 				FlyoutIconDrawerDrawable fid = toolbar.NavigationIcon as FlyoutIconDrawerDrawable;
 				Drawable customIcon;
 
 				if (fid?.IconBitmapSource == image)
+				{
 					customIcon = fid.IconBitmap;
+				}
 				else
+				{
 					customIcon = (await image.GetPlatformImageAsync(MauiContext))?.Value;
 
+					// Fragment might have been disposed while we were waiting for the image drawable
+					if (_disposed)
+					{
+						return;
+					}
+				}
+
 				if (customIcon != null)
 				{
 					if (fid == null)
@@ -438,7 +457,7 @@ protected virtual async void UpdateLeftBarButtonItem(Context context, AToolbar t
 			}
 
 			if (icon != null)
-				icon.Progress = (CanNavigateBack) ? 1 : 0;
+				icon.Progress = CanNavigateBack ? 1 : 0;
 
 			if (command != null || CanNavigateBack)
 			{

src/Controls/tests/TestCases.HostApp/Issues/Issue14801.cs

@@ -0,0 +1,74 @@
+namespace Maui.Controls.Sample.Issues
+{
+	[Issue(IssueTracker.Github, 14801, "NullReferenceException in UpdateLeftBarButtonItem", PlatformAffected.Android)]
+	public class Issue14801 : Shell
+	{
+		public Issue14801()
+		{
+			var mainContent = new ShellContent
+			{
+				ContentTemplate = new DataTemplate(() => new InitialPage()),
+				Title = "Initial",
+				Route = "initial"
+			};
+
+			Items.Add(mainContent);
+			Routing.RegisterRoute("Issue14801_child", typeof(ChildPage));
+		}
+	}
+
+	file class InitialPage : ContentPage
+	{
+		public InitialPage()
+		{
+			Padding = 24;
+			Content = new Button
+			{
+				Text = "Go to child page",
+				AutomationId = "goToChildPage",
+				VerticalOptions = LayoutOptions.Start,
+				Command = new Command(() => Shell.Current.GoToAsync("Issue14801_child"))
+			};
+		}
+	}
+
+	file class ChildPage : ContentPage
+	{
+		public ChildPage()
+		{
+			Shell.SetBackButtonBehavior(this, new BackButtonBehavior
+			{
+				IconOverride = new StreamImageSource
+				{
+					Stream = async _ =>
+					{
+						// Wait until page is popped
+						while (Parent != null)
+						{
+							await Task.Delay(60);
+						}
+
+						// Give it time to finish the navigation
+						await Task.Delay(200);
+
+						// Return the image
+						var imageBytes = Convert.FromBase64String(IconB64);
+						return new MemoryStream(imageBytes);
+					}
+				}
+			});
+
+			Padding = 24;
+			Content = new Button
+			{
+				Text = "Go back",
+				AutomationId = "goBack",
+				VerticalOptions = LayoutOptions.Start,
+				Command = new Command(() => Shell.Current.GoToAsync(".."))
+			};
+		}
+
+		const string IconB64 =
+			"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";
+	}
+}

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Issue14801.cs

@@ -0,0 +1,36 @@
+#if ANDROID
+using NUnit.Framework;
+using UITest.Appium;
+using UITest.Core;
+
+namespace Microsoft.Maui.TestCases.Tests.Issues
+{
+	public class Issue14801 : _IssuesUITest
+	{
+		public override string Issue => "NullReferenceException in UpdateLeftBarButtonItem";
+
+		public Issue14801(TestDevice device) : base(device)
+		{
+		}
+
+		[Test]
+		[Category(UITestCategories.CarouselView)]
+		public async Task PopShellPageBeforeBackImageIsReady()
+		{
+			App.WaitForElement("goToChildPage");
+			App.Tap("goToChildPage");
+
+			// Look for `Shell.SetBackButtonBehavior` in
+			// src/Controls/tests/TestCases.HostApp/Issues/Issue14801.cs
+			App.WaitForElement("goBack");
+			App.Tap("goBack");
+
+			// Wait for main page to be visible
+			App.WaitForElement("goToChildPage");
+
+			// A crash may happen briefly, await for a while to make sure it doesn't happen
+			await Task.Delay(1000);
+		}
+	}
+}
+#endif