FileTransfer下载返回错误,所有属性为空

Ionic FileTransfer ,但它总是返回以下响应:

附言:是的 是下载的直接链接

我真的不知道这个代码有什么问题

{ 代码:空, 来源:空, 目标:空

我的代码:

    const storageDirectory = '';
    this.platform.ready().then(() => {
      storageDirectory = cordova.file.externalRootDirectory;

      let url = this.apiURL + itemId
      const fileTransfer: FileTransferObject = this.transfer.create(); 

      fileTransfer.download(url, storageDirectory + "item" + itemId+ ".xml")
        .then(response => { 
          const alertSuccess = this.alertCtrl.create({
            title: `Successfully downloaded!`,
            subTitle: `The file is in: \n: ${response.toURL()}`,
            buttons: ['Ok']
          }); 
          alertSuccess.present();
        })
        .catch(err => { 
          const alert = this.alertCtrl.create({
            title: 'Error!',
            subTitle: 'Try again!',
            buttons: [{
              text: 'OK',
              handler: () => {
                this.util.getPermission();
              }
            }]
          });
 alert.present();         
 });
 })
}