O elemento raiz do documento não oferece suporte para o designer visual

When you open a XAML file in the WPF Designer for Visual Studio, you might not see any objects in Design view. This can occur in the following cases:

You open a blank XAML file.
You open the Application.xaml file in a Visual Basic project or the App.xaml file in a C# project.
You open a XAML file that contains a root element that is not supported by the designer.

Observação

The WPF Designer supports only Panel, Page, ContentControl and their derivatives as the root element.
You open a XAML file that contains a syntax error in the root element.