I have updated Visual Studio 2019 to version 16.3 which can support .Net Core 3 stable version.
After creating the first WPF app supported by .Net Core 3, the designer is inactive while the XAML designer is enabled in Tools=>Option=>XAML Designer.
How is it possible to activate the UI Designer in Visual Studio 16.3?
An image describing the problem visually has been attached to the question.