Android : Google API - NULL Pointer by getLastKnownLocation

Yesterdy my App worked perfectly but this morning it began to crash. I think the trigger was that I rebooted my Smartphone , so my APP loosed the GPS datas. I started the Debug mode and looked step by step for NULL values and I actuelly find it by this command :

    myLocation = locationManager.getLastKnownLocation(provider);

This returns a NULL object which cause to an crash when I try to use these methods :

latitude = myLocation.getLatitude();
longtitude = myLocation.getLongitude();

How can I make that my Location is not NULL in this case ? I tried to use the NETWORK_PROVIDER but then I have to use the WLAN and this is not the best way I think.Can I still catch informations with the GPS_PROVIDER without to get into a other Application which save them ?

BTW here is my Code :

public class gmapps extends FragmentActivity {

private GoogleMap googleMap;
TextView txt_street , txt_city , txt_country ;
Geocoder geocoder;
List<Address> addresses;
ImageView btn_yes;
Location myLocation;
String address , city , country;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.map);
    txt_street = (TextView)findViewById(R.id.txt_coordinates);
    txt_city = (TextView)findViewById(R.id.txt_city);
    txt_country = (TextView)findViewById(R.id.txt_country);
    btn_yes = (ImageView)findViewById(R.id.btn_yes);

    setUpIfNeeded();

}

private void setUpIfNeeded() {
    if (googleMap == null)
    {
        try{
        googleMap = ((SupportMapFragment) getSupportFragmentManager().findFragmentById(R.id.map)).getMap();
    }catch(Exception e){}

    if (googleMap != null)
    {
        setUpMap();
    }

  }
}

private void setUpMap() {

    googleMap.setMyLocationEnabled(true);

    LocationManager locationManager = (LocationManager) getSystemService(LOCATION_SERVICE);
    Criteria criteria = new Criteria();
    String provider = locationManager.getBestProvider(criteria , true);

        myLocation = locationManager.getLastKnownLocation(provider);                // Here I get the NULL

    googleMap.setMapType(GoogleMap.MAP_TYPE_HYBRID);
    double latitude = myLocation.getLatitude();
    double longtitude = myLocation.getLongitude();
    LatLng latLng = new LatLng(latitude , longtitude);
    googleMap.moveCamera(CameraUpdateFactory.newLatLng(latLng));

    googleMap.animateCamera(CameraUpdateFactory.zoomTo(17));


    try {
        convert_adresses(latitude,longtitude);
    } catch (IOException e) {
        e.printStackTrace();
    }

    txt_street.setText(address);
    txt_city.setText(city);
    txt_country.setText(country);

}

public void convert_adresses (double lat , double lng) throws IOException
{
    geocoder = new Geocoder(this, Locale.getDefault());
    addresses = geocoder.getFromLocation(lat, lng, 1);

    address = addresses.get(0).getAddressLine(0);
    city = addresses.get(0).getAddressLine(1);
    country = addresses.get(0).getAddressLine(2);
}

Thanks everyone !