Prevent leaving through internal method

Ionic Framework ionic-v3
1

Hi there.
If my app pushs a new Page to the screen, there is automatically added an left pointing arrow for leaving.
Now I want to prevent the User from leaving the screen if some of his form inputs are out of range.

ionViewCanLeave(): boolean {
    if(!this.dailyHours.valid) {
      this.alertCtrl.create({
        title: 'Wirklich verlassen?',
        message: 'Deine Studenanzahl liegt nicht im Rahmen von 6 bis 10 Stunden. Die Änderung wird nicht gespeichert. Möchtest du fortfahren?',
        buttons: [
          {
            text: 'Ja',
            handler: () => {
              return true;
            }
          },
          {
            text: 'Nein',
            handler: () => {
              return false;
            }
          }
        ]
      }).present();
    } else {
      return true;
    }
  }

The alert is shown correctly, but the app page is closing in background.
Thats my first problem. My second problem is, that the alert only closes when I click on “Ja”. If I select “Nein” nothing happens.

I hope someone can help me.