Ionic-v4 > Background Geolocation > andriod > app crash when start() is called

I am using ionic v4(angular) beta along with Ionic native background geolocation. However, on android, the app crashes as soon as I call the this.backgroundGeolocation.start();

is anyone here had the same experience? I was just using the example from https://ionicframework.com/docs/native/background-geolocation/

Please help. thanks in advance.