Defer TreeViewItem creation to TreeView.

Fixes nodes not being selected correctly when parent node is expanded.

src/Avalonia.Controls/TreeViewItem.cs

@@ -90,8 +90,20 @@ namespace Avalonia.Controls
 
         internal TreeView? TreeViewOwner => _treeView;
 
-        protected internal override Control CreateContainerForItemOverride() => new TreeViewItem();
-        protected internal override bool IsItemItsOwnContainerOverride(Control item) => item is TreeViewItem;
+        protected internal override Control CreateContainerForItemOverride()
+        {
+            return EnsureTreeView().CreateContainerForItemOverride();
+        }
+
+        protected internal override bool IsItemItsOwnContainerOverride(Control item)
+        {
+            return EnsureTreeView().IsItemItsOwnContainerOverride(item);
+        }
+
+        protected internal override void PrepareContainerForItemOverride(Control container, object? item, int index)
+        {
+            EnsureTreeView().PrepareContainerForItemOverride(container, item, index);
+        }
 
         /// <inheritdoc/>
         protected override void OnAttachedToLogicalTree(LogicalTreeAttachmentEventArgs e)
@@ -283,6 +295,9 @@ namespace Avalonia.Controls
             return logical != null ? result : @default;
         }
 
+        private TreeView EnsureTreeView() => _treeView ??
+            throw new InvalidOperationException("The TreeViewItem is not part of a TreeView.");
+
         private void HeaderDoubleTapped(object? sender, TappedEventArgs e)
         {
             OnHeaderDoubleTapped(e);