We need the System.Xaml Assembly also in the WpfTypefinder because othrwise Types like

NullExtension ({x:Null}) are not found because they life in this Assembly!

src/AddIns/DisplayBindings/WpfDesign/WpfDesign.XamlDom/Project/XamlTypeFinder.cs

@@ -6,6 +6,8 @@ using System.Collections.Generic;
 using System.Diagnostics;
 using System.Reflection;
 using System.Windows.Markup;
+using System.Xaml;
+using XamlReader = System.Windows.Markup.XamlReader;
 
 namespace ICSharpCode.WpfDesign.XamlDom
 {
@@ -268,6 +270,7 @@ namespace ICSharpCode.WpfDesign.XamlDom
 				Instance.RegisterAssembly(typeof(MarkupExtension).Assembly); // WindowsBase
 				Instance.RegisterAssembly(typeof(IAddChild).Assembly); // PresentationCore
 				Instance.RegisterAssembly(typeof(XamlReader).Assembly); // PresentationFramework
+				Instance.RegisterAssembly(typeof(XamlType).Assembly); // System.Xaml
 			}
 		}
 	}