在“browser.on”上抛出未定义的错误。

public openWithInAppBrowser(url: string) {
    let target = "_blank";
    var browser = this.theInAppBrowser.create(url, target, this.options);

    browser.on('loadstart').subscribe(event => {
      console.log(event.url);
    });  
}

构造函数代码包含：

constructor(public storage: Storage, private router: Router, private navCtrl: NavController, private theInAppBrowser: InAppBrowser,
          private toastCtrl: ToastController)