get curent GPS location

Hello!
I need to display current user location in my MapFragment.

I try to use this code found on net:

        LocationManager locationManager;
        locationManager = (LocationManager)getSystemService(LOCATION_SERVICE);
        Criteria criteria = new Criteria();
        String provider = locationManager.getBestProvider(criteria, true);
        Location location = locationManager.getLastKnownLocation(provider);
        locationManager.requestLocationUpdates(provider, 20000, 0, this);

but my application crash. He stop running before displaying anything.

The problem is here :

        Location location = locationManager.getLastKnownLocation(provider);
        locationManager.requestLocationUpdates(provider, 20000, 0, this);

What I do wrong? This is my first android application.