Onesignal receive notification but alert box not working

I am using the latest ionic cli: 6.12.1
I am using the code for onesignal push notification:

setupPush() {
    // I recommend to put these into your environment.ts
    this.oneSignal.startInit('xxxxxxx-xxxxxxx-xxxxxxxx-xxxxxxx-xxxxxxxx', '11234567890');


    // Notifcation was received in general
    this.oneSignal.handleNotificationReceived().subscribe(data => {
      let msg = data.payload.body;
      let title = data.payload.title;
      let additionalData = data.payload.additionalData;
      this.showAlert(title, msg, additionalData.task);

    // Notification was really clicked/opened
    this.oneSignal.handleNotificationOpened().subscribe(data => {
      // Just a note that the data is a different place here!
      let additionalData = data.notification.payload.additionalData;

      this.showAlert('Notification opened', 'You already read this before', additionalData.task);


  async showAlert(title, msg, task) {
    const alert = await this.alertCtrl.create({
      header: title,
      subHeader: msg,
      buttons: [
          text: `Action: ${task}`,
          handler: () => {
            // E.g: Navigate to a specific screen

The message is received, when click the message, it opens the app but the alert is not shown.

Need help, thans in advance.