AvaloniaでSystem.InvalidOperationException: Control 'XXX' is a top level control and cannot be added as a child.

System.InvalidOperationException: Control 'XXX' is a top level control and cannot be added as a child.
   at Avalonia.Controls.TopLevel.OnAttachedToVisualTree(VisualTreeAttachmentEventArgs e)
   at Avalonia.Visual.OnAttachedToVisualTreeCore(VisualTreeAttachmentEventArgs e)
   at Avalonia.Layout.Layoutable.OnAttachedToVisualTreeCore(VisualTreeAttachmentEventArgs e)
   at Avalonia.Input.InputElement.OnAttachedToVisualTreeCore(VisualTreeAttachmentEventArgs e)
   at Avalonia.Controls.Control.OnAttachedToVisualTreeCore(VisualTreeAttachmentEventArgs e)
   at Avalonia.Visual.OnAttachedToVisualTreeCore(VisualTreeAttachmentEventArgs e)
   at Avalonia.Layout.Layoutable.OnAttachedToVisualTreeCore(VisualTreeAttachmentEventArgs e)
   at Avalonia.Input.InputElement.OnAttachedToVisualTreeCore(VisualTreeAttachmentEventArgs e)
   at Avalonia.Controls.Control.OnAttachedToVisualTreeCore(VisualTreeAttachmentEventArgs e)
   at Avalonia.Visual.SetVisualParent(Visual value)
   at Avalonia.Visual.SetVisualParent(IList children, Visual parent)
   at Avalonia.Visual.VisualChildrenChanged(Object sender, NotifyCollectionChangedEventArgs e)
   at Avalonia.Collections.AvaloniaList`1.NotifyAdd(T item, Int32 index)
   at Avalonia.Collections.AvaloniaList`1.Add(T item)
   at Avalonia.Controls.Presenters.ContentPresenter.UpdateChild(Object content)
   at Avalonia.Controls.Presenters.ContentPresenter.UpdateChild()
   at Avalonia.Controls.Presenters.ContentPresenter.ApplyTemplate()
   at Avalonia.Layout.Layoutable.MeasureCore(Size availableSize)
   at Avalonia.Layout.Layoutable.Measure(Size availableSize)
   at Avalonia.Layout.LayoutHelper.MeasureChild(Layoutable control, Size availableSize, Thickness padding)
   at Avalonia.Controls.Decorator.MeasureOverride(Size availableSize)
   at Avalonia.Controls.Primitives.VisualLayerManager.MeasureOverride(Size availableSize)
   at Avalonia.Layout.Layoutable.MeasureCore(Size availableSize)
   at Avalonia.Layout.Layoutable.Measure(Size availableSize)
   at Avalonia.Layout.Layoutable.MeasureOverride(Size availableSize)
   at Avalonia.Layout.Layoutable.MeasureCore(Size availableSize)
   at Avalonia.Layout.Layoutable.Measure(Size availableSize)
   at Avalonia.Layout.Layoutable.MeasureOverride(Size availableSize)
   at Avalonia.Controls.Window.MeasureOverride(Size availableSize)
   at Avalonia.Controls.WindowBase.MeasureCore(Size availableSize)
   at Avalonia.Layout.Layoutable.Measure(Size availableSize)
   at Avalonia.Layout.LayoutManager.Measure(Layoutable control)
   at Avalonia.Layout.LayoutManager.ExecuteInitialLayoutPass()

なんか変なエラーが出たと思ったら、単にWindowの中にWindowを宣言していたからでした。

<Window>
  <Grid>
    <CustomWindow/>
  </Grid>
</Window>