MapView.onMapReady never called in Fragment for load Google Map in MapView

I'll try to show a map in my Android application on a fragment named RoeteFragment. If I debug my code I see that the method onMapReady is never called so that the map will not load.

The fragment implements OnMapReadyCallback like needed and in the onCreateView I get the MapView and call getMapAsync. If I place a breakpoint on that method, it will always been hit, the breakpoint inside onMapReady never been hit. There is no exception thrown.

I've also a valid Google Maps API key in the file res/values/google_maps_api.xml.

Here is my code:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <com.google.android.gms.maps.MapView
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:id="@+id/map" />

</RelativeLayout>

public class RoeteFragment extends Fragment implements OnMapReadyCallback {

    private MapView mapView;
    private static Roete _roete;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.fragment_roete, container, false);
        mapView = (MapView) view.findViewById(R.id.map);
        mapView.getMapAsync(this);
        return view;
    }

    @Override
    public void onMapReady(GoogleMap googleMap) {

        if (_roete != null && _roete.getAankomstLocatie() != null && _roete.getVertrekLocatie() != null) {
            LatLng aankomst = new LatLng(_roete.getAankomstLocatie().getLatitude(), _roete.getAankomstLocatie().getLongitude());
            googleMap.addMarker(new MarkerOptions().position(aankomst).title("aankomst"));
            googleMap.moveCamera(CameraUpdateFactory.newLatLng(aankomst));

            LatLng vertrek = new LatLng(_roete.getVertrekLocatie().getLatitude(), _roete.getVertrekLocatie().getLongitude());
            googleMap.addMarker(new MarkerOptions().position(vertrek).title("vertrek"));
            googleMap.moveCamera(CameraUpdateFactory.newLatLng(vertrek));
        }
    }

    public static Fragment newInstance() {
        return new RoeteFragment ();
    }

    public static Fragment newInstance(Roete roete) {
        _roete = roete;
        return newInstance();
    }
}

Could you file the bug in my code?