打开MainPage.xaml文件,请问到底这是怎么回事？

System.ArgumentNullException
Value cannot be null.
Parameter name: parentContext
   at Microsoft.Windows.Design.Metadata.ReflectionMetadataContext..ctor(IMetadataContext parentContext)
   at MS.Internal.Host.ProjectMetadataContext..ctor(IMetadataContext platformMetadata, AssemblyReferenceProvider referenceProvider)
   at MS.Internal.Host.ProjectMetadataContext.FromReferences(AssemblyReferenceProvider referenceProvider)
   at MS.Internal.Designer.VSDesigner.GetMetadataForDesignerContext(DesignerContext designerContext)
   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()