如何获取WindowStage实例下的主窗口？

onWindowStageCreate(windowStage: window.WindowStage): void {
   windowStage.loadContent('pages/Index', this.storage);
  let windowClass: window.Window | undefined = undefined;
  let promise = windowStage.getMainWindow();//获取主窗口
  promise.then((data) => {
    windowClass = data;
    console.info('Succeeded in obtaining the main window. Data: ' + JSON.stringify(data));
  }).catch((err: BusinessError) => {
    console.error('Failed to obtain the main window. Cause: ' + JSON.stringify(err));
  });
}