Update TreeView index when not attached to logical tree.

Fixes #3551.

src/Avalonia.Controls/TreeViewItem.cs

@@ -51,6 +51,7 @@ namespace Avalonia.Controls
             SelectableMixin.Attach<TreeViewItem>(IsSelectedProperty);
             FocusableProperty.OverrideDefaultValue<TreeViewItem>(true);
             ItemsPanelProperty.OverrideDefaultValue<TreeViewItem>(DefaultPanel);
+            ParentProperty.Changed.AddClassHandler<TreeViewItem>((o, e) => o.OnParentChanged(e));
             RequestBringIntoViewEvent.AddClassHandler<TreeViewItem>((x, e) => x.OnRequestBringIntoView(e));
         }
 
@@ -179,5 +180,16 @@ namespace Avalonia.Controls
 
             return logical != null ? result : @default;
         }
+
+        private void OnParentChanged(AvaloniaPropertyChangedEventArgs e)
+        {
+            if (!((ILogical)this).IsAttachedToLogicalTree && e.NewValue is null)
+            {
+                // If we're not attached to the logical tree, then OnDetachedFromLogicalTree isn't going to be
+                // called when the item is removed. This results in the item not being removed from the index,
+                // causing #3551. In this case, update the index when Parent is changed to null.
+                ItemContainerGenerator.UpdateIndex();
+            }
+        }
     }
 }