It took me a bit of time to debug this so I thought I'd save you all the time. The static constructor in RibbonWindow relies on the Application.Resources being available. That means you need to create an Application object before creating a RibbonWindow.
"The type initializer for
'Divelements.SandRibbon.RibbonWindow' threw an exception: The URI
prefix is not recognized."
at System.Net.WebRequest.Create(Uri requestUri,
Boolean useUriBase)
at System.Net.WebRequest.Create(Uri requestUri)
at MS.Internal.WpfWebRequestHelper.CreateRequest(Uri uri)
at
System.IO.Packaging.PackWebRequest.GetRequest(Boolean allowPseudoRequest)
at System.IO.Packaging.PackWebRequest.GetResponse()
at
MS.Internal.WpfWebRequestHelper.GetResponse(WebRequest request)
at
MS.Internal.WpfWebRequestHelper.GetResponseStream(WebRequest request,
ContentType& contentType)
at
System.Windows.ResourceDictionary.set_Source(Uri value)
at
Divelements.SandRibbon.RibbonWindow..cctor()