message: Fix a crash when the theme doesn't have a `window`
  From team sync today.
diff: @@ -1352,7 +1352,7 @@ void AppHost::_updateTheme()
  const auto b = _logic.TitlebarBrush();
  const auto opacity = b ? ThemeColor::ColorFromBrush(b).A / 255.0 : 0.0;
  - _window->UseMica(theme.Window().UseMica(), opacity);
  + _window->UseMica(theme.Window() ? theme.Window().UseMica() : false, opacity);
  }
  void AppHost::_HandleSettingsChanged(const winrt::Windows::Foundation::IInspectable& /*sender*/,
  