Prepare new containers before adding to tree.

Prevents compiled bindings failing because the `DataContext` is not yet set and is still inheriting the parent `DataContext`.

src/Avalonia.Controls/ItemsControl.cs

@@ -313,9 +313,9 @@ namespace Avalonia.Controls
                 hic.Header = item;
                 hic.HeaderTemplate = itemTemplate;
 
-                var treeTemplate = (itemTemplate ?? hic.FindDataTemplate(item)) as ITreeDataTemplate;
+                itemTemplate ??= hic.FindDataTemplate(item) ?? this.FindDataTemplate(item);
 
-                if (treeTemplate is not null)
+                if (itemTemplate is ITreeDataTemplate treeTemplate)
                 {
                     if (item is not null && treeTemplate.ItemsSelector(item) is { } itemsBinding)
                         BindingOperations.Apply(hic, ItemsProperty, itemsBinding, null);
@@ -481,14 +481,6 @@ namespace Avalonia.Controls
 
         internal void PrepareItemContainer(Control container, object? item, int index)
         {
-            // Putting this precondition in place in case we want to raise an event when a
-            // container is realized. If we want to do that, then the event subscriber will expect
-            // the container to be attached to the tree. Not using IsAttachedToVisualTree here
-            // because a bunch of tests don't have a rooted visual tree.
-            if (container.GetVisualParent() is null)
-                throw new InvalidOperationException(
-                    "Container must be attached to parent before PrepareItemContainer is called.");
-
             var itemContainerTheme = ItemContainerTheme;
 
             if (itemContainerTheme is not null && 

src/Avalonia.Controls/Presenters/PanelContainerGenerator.cs

@@ -137,9 +137,9 @@ namespace Avalonia.Controls.Presenters
                 container = generator.CreateContainer();
             }
 
+            generator.PrepareItemContainer(container, item, index);
             itemsControl.AddLogicalChild(container);
             children.Insert(index, container);
-            generator.PrepareItemContainer(container, item, index);
         }
 
         private void ClearItemsControlLogicalChildren()

src/Avalonia.Controls/VirtualizingCarouselPanel.cs

@@ -275,8 +275,8 @@ namespace Avalonia.Controls
                 }
                 else if (generator.IsItemItsOwnContainer(controlItem))
                 {
-                    AddInternalChild(controlItem);
                     generator.PrepareItemContainer(controlItem, controlItem, index);
+                    AddInternalChild(controlItem);
                     controlItem.SetValue(ItemIsOwnContainerProperty, true);
                     return controlItem;
                 }
@@ -311,8 +311,8 @@ namespace Avalonia.Controls
             var item = items[index];
             var container = generator.CreateContainer();
 
-            AddInternalChild(container);
             generator.PrepareItemContainer(container, item, index);
+            AddInternalChild(container);
 
             return container;
         }