Handle null content in TabControl.

src/Perspex.Controls/ItemsControl.cs

@@ -218,7 +218,7 @@ namespace Perspex.Controls
             {
                 // If the item is its own container, then it will be added to the logical tree when
                 // it was added to the Items collection.
-                if (container.ContainerControl != container.Item)
+                if (container.ContainerControl != null && container.ContainerControl != container.Item)
                 {
                     toAdd.Add(container.ContainerControl);
                 }

src/Perspex.Controls/Primitives/SelectingItemsControl.cs

@@ -323,7 +323,11 @@ namespace Perspex.Controls.Primitives
                      selectedIndex < e.StartingIndex + e.Containers.Count)
             {
                 var container = e.Containers[selectedIndex - e.StartingIndex];
-                MarkContainerSelected(container.ContainerControl, true);
+
+                if (container.ContainerControl != null)
+                {
+                    MarkContainerSelected(container.ContainerControl, true);
+                }
             }
         }
 
@@ -656,7 +660,7 @@ namespace Perspex.Controls.Primitives
 
                         foreach (var item in generator.Containers)
                         {
-                            if (item != null)
+                            if (item?.ContainerControl != null)
                             {
                                 if (MarkContainerSelected(item.ContainerControl, false))
                                 {

tests/Perspex.Controls.UnitTests/TabControlTests.cs

@@ -239,6 +239,30 @@ namespace Perspex.Controls.UnitTests
             Assert.Equal(new object[] { string.Empty, string.Empty }, result);
         }
 
+        [Fact]
+        public void Should_Handle_Changing_To_TabItem_With_Null_Content()
+        {
+            TabControl target = new TabControl
+            {
+                Template = new FuncControlTemplate<TabControl>(CreateTabControlTemplate),
+                Items = new[]
+                {
+                    new TabItem { Header = "Foo" },
+                    new TabItem { Header = "Foo", Content = new Decorator() },
+                    new TabItem { Header = "Baz" },
+                },
+            };
+
+            ApplyTemplate(target);
+
+            target.SelectedIndex = 2;
+
+            var carousel = (Carousel)target.Pages;
+            var page = (TabItem)carousel.SelectedItem;
+
+            Assert.Null(page.Content);
+        }
+
         private Control CreateTabControlTemplate(TabControl parent)
         {
             return new StackPanel