System.NullReferenceException未将对象引用设置到对象的实例。

求解什么错误：  
 System.NullReferenceException
未将对象引用设置到对象的实例。
   在 Microsoft.Windows.Design.Platform.SilverlightMetadataContext.SilverlightXamlExtensionImplementations.<GetXmlNamespaceCompatibilityMappings>d__8.MoveNext()
   在 MS.Internal.Design.Metadata.ReflectionProjectNode.BuildSubsumption()
   在 MS.Internal.Design.Metadata.ReflectionProjectNode.SubsumingNamespace(Identifier identifier)
   在 MS.Internal.Design.Markup.XmlElement.BuildScope(PrefixScope parentScope, IParseContext context)
   在 MS.Internal.Design.Markup.XmlElement.ConvertToXaml(XamlElement parent, PrefixScope parentScope, IParseContext context, IMarkupSourceProvider provider)
   在 MS.Internal.Design.DocumentModel.DocumentTrees.Markup.XamlSourceDocument.FullParse(Boolean convertToXamlWithErrors)
   在 MS.Internal.Design.DocumentModel.DocumentTrees.Markup.XamlSourceDocument.get_RootItem()
   在 Microsoft.Windows.Design.DocumentModel.Trees.ModifiableDocumentTree.get_ModifiableRootItem()
   在 Microsoft.Windows.Design.DocumentModel.MarkupDocumentManagerBase.get_LoadState()
   在 MS.Internal.Host.PersistenceSubsystem.Load()
   在 MS.Internal.Host.Designer.Load()
   在 MS.Internal.Designer.VSDesigner.Load()
   在 MS.Internal.Designer.VSIsolatedDesigner.VSIsolatedView.Load()
   在 MS.Internal.Designer.VSIsolatedDesigner.VSIsolatedDesignerFactory.Load(IsolatedView view)
   在 MS.Internal.Host.Isolation.IsolatedDesigner.BootstrapProxy.LoadDesigner(IsolatedDesignerFactory factory, IsolatedView view)
   在 MS.Internal.Host.Isolation.IsolatedDesigner.BootstrapProxy.LoadDesigner(IsolatedDesignerFactory factory, IsolatedView view)
   在 MS.Internal.Host.Isolation.IsolatedDesigner.Load()
   在 MS.Internal.Designer.DesignerPane.LoadDesignerView()