canGoBack always return false

Ionic Framework ionic-v3
1

hello all i have i ion-tabs page as a root page ,
and in my app.component.ts a i register a backbuttonAction for showing alert for the user to confirm the app exit,
this is my code for showing alert

      this.platform.registerBackButtonAction(() => {
        if (this.alertShown == false && !this.nav.canGoBack()) {
          this.presentConfirm();
        } else if (this.alertShown == false) {
          this.nav.pop();
        }
      });
  presentConfirm() {
    let alert = this.alertCtrl.create({
      title: 'تأكيد الخروج',
      message: 'انت علي وشك الخروج من التطبيق ,هل تريد ذالك ؟',
      buttons: [
        {
          text: 'الغاء',
          role: 'cancel',
          handler: () => {
            console.log('Cancel clicked');
            this.alertShown = false;
          }
        },
        {
          text: 'نعم اريد',
          handler: () => {
            console.log('Yes clicked');
            this.platform.exitApp();
          }
        }
      ]
    });
    alert.present().then(() => {
      this.alertShown = true;
    });
  }

my problem here is that can go back always return false , any help please ?
thanks in advance