问为什么fusedLocationProviderClient.getLast位置总是返回null？

EN

FusedLocationProviderClient client;
double latitude, longitude;

private OnMapReadyCallback callback = new OnMapReadyCallback() {

    @Override
    public void onMapReady(GoogleMap googleMap) {

        //Check condition
        if (ContextCompat.checkSelfPermission(getActivity(),
                Manifest.permission.ACCESS_FINE_LOCATION) ==
                PackageManager.PERMISSION_GRANTED &&
                ContextCompat.checkSelfPermission(getActivity(),
                        Manifest.permission.ACCESS_COARSE_LOCATION) ==
                        PackageManager.PERMISSION_GRANTED) {
            //When permission is granted, call method
            getCurrentLocation();
        } else {
            //When permission is not granted, request permission
            requestPermissions(new String[]{Manifest.permission.ACCESS_FINE_LOCATION,
                    Manifest.permission.ACCESS_COARSE_LOCATION}, 100);
        }


        //LatLng object creation
        LatLng latLng = new LatLng(latitude, longitude);

        //Set map coordinates using latLng object
        googleMap.addMarker(new MarkerOptions().position(latLng).title("Current Position"));
        googleMap.moveCamera(CameraUpdateFactory.newLatLng(latLng));
    }
};

@Nullable
@Override
public View onCreateView(@NonNull LayoutInflater inflater,
                         @Nullable ViewGroup container,
                         @Nullable Bundle savedInstanceState) {
    View v = inflater.inflate(R.layout.fragment_maps, container, false);
    return v;
}

@Override
public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
    super.onViewCreated(view, savedInstanceState);
    SupportMapFragment mapFragment =
            (SupportMapFragment) getChildFragmentManager().findFragmentById(R.id.map);
    if (mapFragment != null) {
        mapFragment.getMapAsync(callback);
    }
}


@Override
public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
    super.onRequestPermissionsResult(requestCode, permissions, grantResults);
    //Check condition
    if (requestCode == 100 && (grantResults.length > 0) && (grantResults[0] + grantResults[1] ==
            PackageManager.PERMISSION_GRANTED)) {
        //When permission are granted, call method
        getCurrentLocation();
    } else {
        Toast.makeText(getActivity(), "Permission denied", Toast.LENGTH_SHORT).show();
    }
}

@SuppressLint("MissingPermission")
private void getCurrentLocation() {

    client = LocationServices.getFusedLocationProviderClient(getActivity());        

    //Initialize location manager, whose allow to get periodic updates of the location
    LocationManager locationManager = (LocationManager) getActivity()
            .getSystemService(Context.LOCATION_SERVICE);

    //Check condition, when GPS and NETWORK provider are enabled, WORK
    if (locationManager.isProviderEnabled(LocationManager.GPS_PROVIDER)
            || locationManager.isProviderEnabled(LocationManager.NETWORK_PROVIDER)) {
        //When location is enabled, get last location, THE PROBLEM IS THERE
        client.getLastLocation().addOnCompleteListener(new OnCompleteListener<Location>() {
            @Override
            public void onComplete(@NonNull Task<Location> task) {
                Location location = task.getResult();
                //Check condition
                if (location != null) {
                    //When location result is not null, set latitude and longitude
                    latitude = location.getLatitude();
                    longitude = location.getLongitude();

                } else {
                    //When location result is not null, initialize location request
                    LocationRequest locationRequest = new LocationRequest()
                            .setPriority(LocationRequest.PRIORITY_HIGH_ACCURACY)
                            .setInterval(10000)
                            .setFastestInterval(1000)
                            .setNumUpdates(1);

                    //Initialize location callback
                    LocationCallback locationCallback = new LocationCallback() {
                        @Override
                        public void onLocationResult(@NonNull LocationResult locationResult) {
                            //Initialize location
                            Location location1 = locationResult.getLastLocation();
                            //Set latitude and longitude
                            latitude = location1.getLatitude();
                            longitude = location1.getLatitude();
                        }
                    };
                    //Request location updates
                    client.requestLocationUpdates(locationRequest, locationCallback, Looper.myLooper());
                }
            }
        });
    } else {
        //When location service is not enabled, open location setting
        startActivity(new Intent(Settings.ACTION_LOCATION_SOURCE_SETTINGS)
                .setFlags(Intent.FLAG_ACTIVITY_NEW_TASK));
    }
}