Windows Phone - Caliburn Micro and App.xaml error

When I add Caliburn Micro to a new project, I have always some error in App.xaml.
With Caliburn 1.5.1 I found a new problem: "Object Reference not set to an instance of an object".

This error don't prevent the project build, but it seems related to the xaml.
That's why the RootFrame into designer is null.

The fix is easy! Just open your Bootstrapper and change

container.RegisterPhoneServices(RootFrame);

with..

if (!Execute.InDesignMode)
 container.RegisterPhoneServices(RootFrame);