Merge pull request #2510 from AvaloniaUI/fixes/2507-carousel-exception

Only generate containers on the fly if virtualized.

src/Avalonia.Controls/Presenters/CarouselPresenter.cs

@@ -210,7 +210,7 @@ namespace Avalonia.Controls.Presenters
         {
             var container = ItemContainerGenerator.ContainerFromIndex(index);
 
-            if (container == null)
+            if (container == null && IsVirtualized)
             {
                 var item = Items.Cast<object>().ElementAt(index);
                 var materialized = ItemContainerGenerator.Materialize(index, item, MemberSelector);

tests/Avalonia.Controls.UnitTests/CarouselTests.cs

@@ -152,6 +152,29 @@ namespace Avalonia.Controls.UnitTests
             Assert.Equal("Bar", ((TextBlock)child).Text);
         }
 
+        [Fact]
+        public void Selected_Item_Changes_To_First_Item_When_Item_Added()
+        {
+            var items = new ObservableCollection<string>();
+            var target = new Carousel
+            {
+                Template = new FuncControlTemplate<Carousel>(CreateTemplate),
+                Items = items,
+                IsVirtualized = false
+            };
+
+            target.ApplyTemplate();
+            target.Presenter.ApplyTemplate();
+
+            Assert.Equal(-1, target.SelectedIndex);
+            Assert.Empty(target.GetLogicalChildren());
+
+            items.Add("Foo");
+
+            Assert.Equal(0, target.SelectedIndex);
+            Assert.Single(target.GetLogicalChildren());
+        }
+
         [Fact]
         public void Selected_Index_Changes_To_When_Items_Assigned_Null()
         {