[iOS] Fix for NullReferenceException in WillMoveToParentViewController When Removing Page During Navigation

src/Controls/src/Core/Compatibility/Handlers/NavigationPage/iOS/NavigationRenderer.cs

@@ -1322,6 +1322,11 @@ public override void WillMoveToParentViewController(UIViewController parent)
 			{
 				base.WillMoveToParentViewController(parent);
 
+				if (_tracker is null)
+				{
+					return;
+				}
+
 				if (parent is null)
 				{
 					_tracker.CollectionChanged -= TrackerOnCollectionChanged;

src/Controls/tests/TestCases.HostApp/Issues/Issue25726.cs

@@ -0,0 +1,62 @@
+namespace Maui.Controls.Sample.Issues;
+
+[XamlCompilation(XamlCompilationOptions.Compile)]
+[Issue(IssueTracker.Github, 25726, "NullReferenceException in WillMoveToParentViewController When Removing Page During Navigation on iOS", PlatformAffected.iOS)]
+public partial class Issue25726 : NavigationPage
+{
+    public Issue25726()
+    {
+        // Directly push Page 1
+        PushAsync(CreatePage("Page 1", "Navigate to Page 2", "NavigateToPage2Button", CreatePage2));
+    }
+
+    // Common method to create pages with button and navigation functionality
+    private ContentPage CreatePage(string title, string buttonText, string automationId, Func<ContentPage> nextPageCreator)
+    {
+        return new ContentPage
+        {
+            Title = title,
+            Content = new Button
+            {
+                Text = buttonText,
+                AutomationId = automationId,
+                Command = new Command(async () =>
+                {
+                    var nextPage = nextPageCreator();
+                    await PushAsync(nextPage);
+                })
+            }
+        };
+    }
+
+    private ContentPage CreatePage2()
+    {
+        var page2 = CreatePage("Page 2", "Navigate to Page 3", "NavigateToPage3Button", CreatePage3);
+
+
+        page2.Content = new Button
+        {
+            Text = "Navigate to Page 3",
+            AutomationId = "NavigateToPage3Button",
+            Command = new Command(() =>
+            {
+                var page3 = CreatePage3();
+                PushAsync(page3);
+
+                // Manipulate the stack after navigating to Page 3
+                Navigation.RemovePage(Navigation.NavigationStack[^2]);
+            })
+        };
+
+        return page2;
+    }
+
+    private ContentPage CreatePage3()
+    {
+        return new ContentPage
+        {
+            Title = "Page 3",
+            Content = new Label { Text = "This is Page 3", AutomationId = "Page3Label" }
+        };
+    }
+}

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Issue25726.cs

@@ -0,0 +1,38 @@
+using NUnit.Framework;
+using UITest.Appium;
+using UITest.Core;
+
+namespace Microsoft.Maui.TestCases.Tests.Issues;
+
+internal class Issue25726 : _IssuesUITest
+{
+#if ANDROID || WINDOWS
+    private const string BackButtonIdentifier = "";
+#else
+    private const string BackButtonIdentifier = "Page 1";
+#endif
+
+    public Issue25726(TestDevice device) : base(device) { }
+
+    public override string Issue => "NullReferenceException in WillMoveToParentViewController When Removing Page During Navigation on iOS";
+
+    [Test]
+    [Category(UITestCategories.Navigation)]
+    public void NavigationShouldNotCrashWhenRemovingPageDuringNavigation()
+    {
+        App.WaitForElement("NavigateToPage2Button");
+        App.Tap("NavigateToPage2Button");
+
+        App.WaitForElement("NavigateToPage3Button");
+        App.Tap("NavigateToPage3Button");
+
+        // Ensure the label on Page 3 is present to verify navigation completed successfully
+        // If the app crashes during navigation, the test will fail before reaching this line.
+        App.WaitForElement("Page3Label");
+
+        App.TapBackArrow(BackButtonIdentifier);
+
+        // Ensure we navigate back to Page 1 successfully
+        App.WaitForElement("NavigateToPage2Button");
+    }
+}