System.InvalidOperationException: '{DependencyProperty.UnsetValue}'은(는) 'FocusVisualStyle' 속성의 유효한 값이 아닙니다.
  ?, in EffectiveValueEntry DependencyObject.GetEffectiveValue(EntryIndex entryIndex, DependencyProperty dp, RequestFlags requests)
  ?, in EffectiveValueEntry DependencyObject.GetValueEntry(EntryIndex entryIndex, DependencyProperty dp, PropertyMetadata metadata, RequestFlags requests)
  ?, in object DependencyObject.GetValue(DependencyProperty dp)
  ?, in Style FrameworkElement.get_FocusVisualStyle()
  ?, in void KeyboardNavigation.ShowFocusVisual(DependencyObject element)
  ?, in void FrameworkElement.OnGotKeyboardFocus(object sender, KeyboardFocusChangedEventArgs e)
  ?, in void RoutedEventArgs.InvokeHandler(Delegate handler, object target)
  ?, in void EventRoute.InvokeHandlersImpl(object source, RoutedEventArgs args, bool reRaised)
  ?, in void UIElement.RaiseEventImpl(DependencyObject sender, RoutedEventArgs args)
  ?, in void UIElement.RaiseTrustedEvent(RoutedEventArgs args)
  ?, in bool InputManager.ProcessStagingArea()
  ?, in void KeyboardDevice.ChangeFocus(DependencyObject focus, int timestamp)
  ?, in void KeyboardDevice.Focus(DependencyObject focus, bool askOld, bool askNew, bool forceToNullIfFailed) x 2
  ?, in void HwndKeyboardInputProvider.OnSetFocus(IntPtr hwnd)
  ?, in IntPtr HwndKeyboardInputProvider.FilterMessage(IntPtr hwnd, WindowMessage message, IntPtr wParam, IntPtr lParam, ref bool handled)
  ?, in object ExceptionWrapper.InternalRealCall(Delegate callback, object args, int numArgs)
  ?, in object ExceptionWrapper.TryCatchWhen(object source, Delegate callback, object args, int numArgs, Delegate catchHandler)
  ?, in object Dispatcher.LegacyInvokeImpl(DispatcherPriority priority, TimeSpan timeout, Delegate method, object args, int numArgs)
  ?, in IntPtr HwndSubclass.SubclassWndProc(IntPtr hwnd, int msg, IntPtr wParam, IntPtr lParam)