[WInUI] XLS0503 and WMC0020 errors occur when added the "Blank Window (WinUI 3 in Desktop)" into the MainWindow

[XamlTest]

Describe the bug

When added the "Blank Window (WinUI 3 in Desktop)" item into the MainWindow.xaml. The XLS0503 and WMC0020 errors occur.

Steps to reproduce the bug

Affected Branch: main
Affected Build: 35422.220

1.Create a C# Blank App, Package (WinUI3 in Desktop) project.
2.Right-click the Project name (Desktop) project node in Solution Explorer and select Add -> New Item.
3. In the Add New Item dialog box, select the WinUI tab, choose the "Blank Window (WinUI 3 in Desktop)" to add, and then click Add.
4.Build your solution
5.Add the new item created in step3 into MainWindow.xaml

6.Press F5

Expected behavior

The project should be built successfully.

Screenshots

Actual Result:
The project is built failed with XLS0503 & WMC0020 errors.
XLS0503: A value of type 'BlankWindow1' cannot be added to a collection or dictionary of type 'UIElementCollection'.
WMC0020: Cannot add 'BlankWindow1' into the collection property 'Children', type must be 'UIElement'

NuGet package version

None

Packaging type

No response

Windows version

No response

IDE

No response

Additional context

1. This issue also repro on 17.6.21 p1 [35426.19.d17.6]
2. This issue is not repro on WPF Application project.
   

[codendone]

I think this is by design: Window in WinUI3 is not a FrameworkElement.

[github-actions]

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[karkarl]

Closing as it is by design.