I assume VS can't render in design mode custom types like local:PageBase
It might be due to lot of factors: abstract type, there is no default parametreless constructor, errors during resolving dependencies in design time.. and etc.

Try to change in xaml local:PageBase to base type: Page or UserControl(depends on what is your PageBase impelent) and specify the type in x:Class attribute (it should be non abstract with default constructor which will call InitializeComponents or if it custom control - specify DefaultStyleKey)

I assume after succeeded rebuild - you will see the content in design time.