Hi,
Make sure that you have the following things in your xaml file (open with notepad++ or something similar):
The tag:
“mva:VisualBasic.Settings
<x:Null />
</mva:VisualBasic.Settings>”
the NamespacesForImplementation:
“<x:String>System.Web</x:String>”
and the reference added:
“< AssemblyReference>System.Web</ AssemblyReference>” (lose the spaces 
)
It should work after reopening the file in Studio.
Thank you