Commented Unassigned: Saving file causes VS crash [1600]

Used 1.0 RC build under MSVS2013 Ultimate (Update 3)

```
Application: devenv.exe
Framework Version: v4.0.30319
Description: The process was terminated due to an unhandled exception.
Exception Info: System.ArgumentNullException
Stack:
at System.Runtime.CompilerServices.AsyncMethodBuilderCore.<ThrowAsync>b__4(System.Object)
at System.Windows.Threading.ExceptionWrapper.InternalRealCall(System.Delegate, System.Object, Int32)
at MS.Internal.Threading.ExceptionFilterHelper.TryCatchWhen(System.Object, System.Delegate, System.Object, Int32, System.Delegate)
at System.Windows.Threading.DispatcherOperation.InvokeImpl()
at System.Windows.Threading.DispatcherOperation.InvokeInSecurityContext(System.Object)
at System.Threading.ExecutionContext.RunInternal(System.Threading.ExecutionContext, System.Threading.ContextCallback, System.Object, Boolean)
at System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext, System.Threading.ContextCallback, System.Object, Boolean)
at System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext, System.Threading.ContextCallback, System.Object)
at System.Windows.Threading.DispatcherOperation.Invoke()
at System.Windows.Threading.Dispatcher.ProcessQueue()
at System.Windows.Threading.Dispatcher.WndProcHook(IntPtr, Int32, IntPtr, IntPtr, Boolean ByRef)
at MS.Win32.HwndWrapper.WndProc(IntPtr, Int32, IntPtr, IntPtr, Boolean ByRef)
at MS.Win32.HwndSubclass.DispatcherCallbackOperation(System.Object)
at System.Windows.Threading.ExceptionWrapper.InternalRealCall(System.Delegate, System.Object, Int32)
at MS.Internal.Threading.ExceptionFilterHelper.TryCatchWhen(System.Object, System.Delegate, System.Object, Int32, System.Delegate)
at System.Windows.Threading.Dispatcher.LegacyInvokeImpl(System.Windows.Threading.DispatcherPriority, System.TimeSpan, System.Delegate, System.Object, Int32)
at MS.Win32.HwndSubclass.SubclassWndProc(IntPtr, Int32, IntPtr, IntPtr)
```
Comments: can you pls give us repro steps for this issue? we're having a hard time repro'ing it. thanks!