fix carousel presenter and add unit test for condition.

src/Avalonia.Controls/Presenters/CarouselPresenter.cs

@@ -125,8 +125,22 @@ namespace Avalonia.Controls.Presenters
                         var containers = generator.Containers.ToList();
                         generator.Clear();
                         Panel.Children.RemoveAll(containers.Select(x => x.ContainerControl));
+
+                        var newIndex = SelectedIndex;
+
+                        if(SelectedIndex < 0)
+                        {
+                            if(Items != null && Items.Count() > 0)
+                            {
+                                newIndex = 0;
+                            }
+                            else
+                            {
+                                newIndex = -1;
+                            }
+                        }
                         
-                        MoveToPage(-1, SelectedIndex >= 0 ? SelectedIndex : e.NewStartingIndex);
+                        MoveToPage(-1, newIndex);
                     }
                     break;     
             }

tests/Avalonia.Controls.UnitTests/CarouselTests.cs

@@ -170,6 +170,41 @@ namespace Avalonia.Controls.UnitTests
             Assert.Equal("Bar", ((TextBlock)child).Text);
         }
 
+        [Fact]
+        public void Selected_Index_Changes_To_When_Items_Assigned_Null()
+        {
+            var items = new ObservableCollection<string>
+            {
+               "Foo",
+               "Bar",
+               "FooBar"
+            };
+
+            var target = new Carousel
+            {
+                Template = new FuncControlTemplate<Carousel>(CreateTemplate),
+                Items = items,
+                IsVirtualized = false
+            };
+
+            target.ApplyTemplate();
+            target.Presenter.ApplyTemplate();
+
+            Assert.Single(target.GetLogicalChildren());
+
+            var child = target.GetLogicalChildren().Single();
+
+            Assert.IsType<TextBlock>(child);
+            Assert.Equal("Foo", ((TextBlock)child).Text);
+
+            target.Items = null;
+
+            var numChildren = target.GetLogicalChildren().Count();
+
+            Assert.Equal(0, numChildren);
+            Assert.Equal(-1, target.SelectedIndex);
+        }
+
         [Fact]
         public void Selected_Index_Is_Maintained_Carousel_Created_With_Non_Zero_SelectedIndex()
         {