title: VS2010/Silverlight designer unhandled exception
body: <p>I have VS2010 10.0.30319.1 RTMRel and Silverlight 4 installed.</p>
  
  <p>Let me make it clear that I've worked on several projects without any designer issue. </p>
  
  <p>I don't recall messing with either VS or Silverlight 4, however today when I opened a project and tried to tweak the UI of my web application, the designer wouldn't load. I created a new project, and the same thing happed. Below is the exception. Any help will be appreciated.</p>
  
  <pre><code>System.NullReferenceException
  Object reference not set to an instance of an object.
     at Microsoft.Windows.Design.Platform.SilverlightMetadataContext.SilverlightXamlExtensionImplementations.&lt;GetXmlNamespaceCompatibilityMappings&gt;d__8.MoveNext()
     at MS.Internal.Design.Metadata.ReflectionProjectNode.BuildSubsumption()
     at MS.Internal.Design.Metadata.ReflectionProjectNode.SubsumingNamespace(Identifier identifier)
     at MS.Internal.Design.Markup.XmlElement.BuildScope(PrefixScope parentScope, IParseContext context)
     at MS.Internal.Design.Markup.XmlElement.ConvertToXaml(XamlElement parent, PrefixScope parentScope, IParseContext context, IMarkupSourceProvider provider)
     at MS.Internal.Design.DocumentModel.DocumentTrees.Markup.XamlSourceDocument.FullParse(Boolean convertToXamlWithErrors)
     at MS.Internal.Design.DocumentModel.DocumentTrees.Markup.XamlSourceDocument.get_RootItem()
     at Microsoft.Windows.Design.DocumentModel.Trees.ModifiableDocumentTree.get_ModifiableRootItem()
     at Microsoft.Windows.Design.DocumentModel.MarkupDocumentManagerBase.get_LoadState()
     at MS.Internal.Host.PersistenceSubsystem.Load()
     at MS.Internal.Host.Designer.Load()
     at MS.Internal.Designer.VSDesigner.Load()
     at MS.Internal.Designer.VSIsolatedDesigner.VSIsolatedView.Load()
     at MS.Internal.Designer.VSIsolatedDesigner.VSIsolatedDesignerFactory.Load(IsolatedView view)
     at MS.Internal.Host.Isolation.IsolatedDesigner.BootstrapProxy.LoadDesigner(IsolatedDesignerFactory factory, IsolatedView view)
     at MS.Internal.Host.Isolation.IsolatedDesigner.BootstrapProxy.LoadDesigner(IsolatedDesignerFactory factory, IsolatedView view)
     at MS.Internal.Host.Isolation.IsolatedDesigner.Load()
     at MS.Internal.Designer.DesignerPane.LoadDesignerView()
  </code></pre>