public class MapActivity  extends  FragmentActivity{

    private GoogleMap mMap;
    private ClusterManager<MyItem> mClusterManager;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.map);
        setUpMapIfNeeded();
    }

    @Override
    protected void onResume() {
        super.onResume();
        setUpMapIfNeeded();
    }

    private void setUpMapIfNeeded() {
        if (mMap != null) {
            return;
        }
        mMap = ((SupportMapFragment) getSupportFragmentManager().findFragmentById(R.id.map)).getMap();
        if (mMap != null) {
            //if comment out mCLusterManager instantiation, the app works and show the map
            //mClusterManager = new ClusterManager<MyItem>(this, mMap);

           mMap.moveCamera(CameraUpdateFactory.newLatLngZoom(new LatLng(51.5, -0.35), 9));
        }
    }
}