message: Fix a crash when showTabsInTitlebar:false
  Does what it says on the tin.
diff: @@ -1622,8 +1622,11 @@ void AppHost::_PropertyChangedHandler(const winrt::Windows::Foundation::IInspect
  const winrt::Windows::UI::Xaml::Data::PropertyChangedEventArgs& e)
  {
  if (e.PropertyName() == L"TitlebarBrush")
  + {
  + if (_useNonClientArea)
  {
  auto nonClientWindow{ static_cast<NonClientIslandWindow*>(_window.get()) };
  nonClientWindow->SetTitlebarBackground(_logic.TitlebarBrush());
  }
  }
  +}
  