My custom controls are not loading in VS.NET's designer because of a null reference exception. It's got everything to do with the way I am retrieving the baseUri of the application when it runs in the browser:

_uriPrefix = Application.Current.Host.Source.AbsoluteUri.Substring(0,
     Application.Current.Host.Source.AbsoluteUri.IndexOf("/ClientBin")).Trim();

According to the exception details and the help file I'm directed to (here) I'm not correctly designing my app for a runtime that's not the browser (i.e. the new WPF editor in VS.NET or Expression Blend).

So, the question is how do retrieve the baseUri (the http://localhost:#### part of my application) if I can't use Application.Host which apparently is null during design time? is there a safe way to do this so I can load my custom controls in a designer?