Fixed the NRE in CarouselViewController on iOS 15.5 & 16.4

src/Controls/src/Core/Handlers/Items/iOS/CarouselViewController.cs

@@ -61,20 +61,45 @@ public override UICollectionViewCell GetCell(UICollectionView collectionView, NS
 		{
 			UICollectionViewCell cell;
 
-			if (ItemsView?.Loop == true && _carouselViewLoopManager != null)
-			{
-				var cellAndCorrectedIndex = _carouselViewLoopManager.GetCellAndCorrectIndex(collectionView, indexPath, DetermineCellReuseId(indexPath));
-				cell = cellAndCorrectedIndex.cell;
-				var correctedIndexPath = NSIndexPath.FromRowSection(cellAndCorrectedIndex.correctedIndex, 0);
-
-				if (cell is DefaultCell defaultCell)
+			if (ItemsView?.Loop == true)
+			{
+				// In iOS 15 and 16, when the ItemsSource of the CarouselView is updated from another page
+				// via the ViewModel (or similar), GetCell is called immediately—while _carouselViewLoopManager
+				// is still null. As a result, an invalid index is passed to base.GetCell, leading to an ArgumentNullException.
+				// 
+				// However, in iOS 17 and 18, GetCell is only called after navigating back to the page containing
+				// the CarouselView. By that time, CarouselViewLoopManager has been properly initialized during
+				// the window attachment, so the issue does not occur.
+				// 
+				// This fix ensures proper handling across all iOS versions by initializing the loop manager
+				// when needed and providing a fallback implementation, making navigation scenarios work consistently.
+				if (_carouselViewLoopManager is null)
 				{
-					UpdateDefaultCell(defaultCell, correctedIndexPath);
+					InitializeCarouselViewLoopManager();
 				}
 
-				if (cell is TemplatedCell templatedCell)
+				if (_carouselViewLoopManager is not null)
 				{
-					UpdateTemplatedCell(templatedCell, correctedIndexPath);
+					var cellAndCorrectedIndex = _carouselViewLoopManager.GetCellAndCorrectIndex(collectionView, indexPath, DetermineCellReuseId(indexPath));
+					cell = cellAndCorrectedIndex.cell;
+					var correctedIndexPath = NSIndexPath.FromRowSection(cellAndCorrectedIndex.correctedIndex, 0);
+
+					if (cell is DefaultCell defaultCell)
+					{
+						UpdateDefaultCell(defaultCell, correctedIndexPath);
+					}
+
+					if (cell is TemplatedCell templatedCell)
+					{
+						UpdateTemplatedCell(templatedCell, correctedIndexPath);
+					}
+				}
+				else
+				{
+					// Fallback case: If _carouselViewLoopManager is still null after attempted initialization,
+					// we bypass loop-specific behavior and use base implementation directly.
+
+					cell = base.GetCell(collectionView, indexPath);
 				}
 			}
 			else

src/Controls/tests/TestCases.HostApp/Issues/Issue28557.cs

@@ -0,0 +1,125 @@
+namespace Controls.TestCases.HostApp.Issues;
+
+using Maui.Controls.Sample.Issues;
+using System.Collections.ObjectModel;
+using System.ComponentModel;
+using System.Runtime.CompilerServices;
+
+[Issue(IssueTracker.Github, 28557, "NRE in CarouselViewController on iOS 15.5 & 16.4", PlatformAffected.iOS)]
+public class Issue28557 : TestNavigationPage
+{
+    protected override void Init()
+    {
+        PushAsync(new Issue28557FirstPage());
+    }
+}
+
+public class Issue28557FirstPage : ContentPage
+{
+    private readonly Issue28557ViewModel _model = new();
+    public Issue28557FirstPage()
+    {
+        BindingContext = _model;
+
+        var carouselView = new CarouselView
+        {
+            ItemsSource = _model.Source,
+            AutomationId = "TestCarouselView",
+            ItemsLayout = new LinearItemsLayout(ItemsLayoutOrientation.Horizontal)
+            {
+                SnapPointsType = SnapPointsType.MandatorySingle
+            },
+            ItemTemplate = new DataTemplate(() =>
+            {
+                var label = new Label
+                {
+                    HorizontalOptions = LayoutOptions.Center,
+                    VerticalOptions = LayoutOptions.Center
+                };
+                label.SetBinding(Label.TextProperty, ".");
+                return new Grid { Children = { label } };
+            })
+        };
+
+        var navigateButton = new Button
+        {
+            Text = "Navigate",
+            AutomationId = "NavigateToButton",
+            HorizontalOptions = LayoutOptions.Center,
+            VerticalOptions = LayoutOptions.Center
+        };
+        navigateButton.Clicked += async (s, e) =>
+        {
+            await Navigation.PushAsync(new Issue28557SecondPage(_model));
+        };
+
+        var grid = new Grid
+        {
+            RowDefinitions =
+            {
+                new RowDefinition(GridLength.Star),
+                new RowDefinition(GridLength.Auto)
+            }
+        };
+        grid.Add(carouselView);
+        grid.Add(navigateButton, 0, 1);
+        Content = grid;
+    }
+}
+
+public class Issue28557SecondPage : ContentPage
+{
+    private readonly Issue28557ViewModel _viewModel;
+
+    public Issue28557SecondPage(Issue28557ViewModel viewModel)
+    {
+        _viewModel = viewModel;
+
+        var crashButton = new Button
+        {
+            Text = "CRASH",
+            AutomationId = "SourceUpdateAndNavigateBackButton",
+            HorizontalOptions = LayoutOptions.Center,
+            VerticalOptions = LayoutOptions.Center
+        };
+
+        crashButton.Clicked += async (s, e) =>
+        {
+            if (_viewModel?.Source != null && _viewModel.Source.Count > 0)
+            {
+                _viewModel.Source[0] = "CRASH";
+                await Task.Delay(1000);
+                await Navigation.PopAsync();
+            }
+        };
+        Content = crashButton;
+    }
+}
+public class Issue28557ViewModel : INotifyPropertyChanged
+{
+    private ObservableCollection<string> _source;
+
+    public ObservableCollection<string> Source
+    {
+        get => _source;
+        set
+        {
+            _source = value;
+            OnPropertyChanged();
+        }
+    }
+
+    public Issue28557ViewModel()
+    {
+        _source = new ObservableCollection<string>(["Test1", "Test2", "Test3"]);
+    }
+
+    public event PropertyChangedEventHandler PropertyChanged;
+
+    protected void OnPropertyChanged([CallerMemberName] string propertyName = null)
+    {
+        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
+    }
+}
+
+

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Issue28557.cs

@@ -0,0 +1,25 @@
+#if TEST_FAILS_ON_WINDOWS // https://github.com/dotnet/maui/issues/29245 - test fails on windows due to appium related issues in carouselview
+using NUnit.Framework;
+using UITest.Appium;
+using UITest.Core;
+
+namespace Microsoft.Maui.TestCases.Tests.Issues;
+
+public class Issue28557 : _IssuesUITest
+{
+	public Issue28557(TestDevice device) : base(device) { }
+
+	public override string Issue => "NRE in CarouselViewController on iOS 15.5 & 16.4";
+
+	[Test]
+	[Category(UITestCategories.CarouselView)]
+	public void CarouselViewShouldNotCrashOnSourceUpdateWithPageNavigation()
+	{
+		App.WaitForElement("TestCarouselView");
+		App.Tap("NavigateToButton");
+		App.WaitForElement("SourceUpdateAndNavigateBackButton");
+		App.Tap("SourceUpdateAndNavigateBackButton");
+		App.WaitForElement("TestCarouselView");
+	}
+}
+#endif