InvalidOperationException: Cannot set Visibility or call Show, ShowDialog, or WindowInteropHelper.EnsureHandle after a Window has closed.
StackTrace:
   at System.Windows.Window.VerifyCanShow()
   at System.Windows.Window.CreateSourceWindow(Boolean duringShow)
   at System.Windows.Interop.WindowInteropHelper.EnsureHandle()
   at Quicker.View.UI.WhiteboardWindow.<BtnSave_OnClick>d__31.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
   at System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Int32 numArgs, Delegate catchHandler)