Android :: Why Does LocationListener Become Null?

I am having issues regarding a LocationListener in my Service called myService.

Here is my code:

///onStart method ..
onStart() {
locationManager = (LocationManager) getSystemService(Context.LOCATION_SERVICE)
.
.
provider = locationManager.getBestProvider(criteria, true);
locationListener = (LocationListener) new MyLocationListener();
locationManager.requestLocationUpdates(provider, 0, 0, locationListener);
}

In my Activity there is a button which should stop the service. On the click of the button I am doing:

stopService(new Intent(getApplicationContext(), myService.class).................