У меня ошибка выходит java.lang.string com.google.firebase.auth.firebaseuser.getuid()' что делать?

У меня ошибка выходит java.lang.string com.google.firebase.auth.firebaseuser.getuid()' on a null object reference, я пробовал разные способы но не помогло я не знаю что делать, нажимаю кнопку "выход" и должен на главный меню попасть но вылетает, на фото есть ошибки который у меня выходит, я особо не понимаю программирование надеюсь подробнее объясните, заранее спасибо. https://drive.google.com/file/d/1V7t8AR10yKYYSZEgIeYIidw1LQRoNv1Z/view?usp=drivesdk

public class MapDriverActivity extends FragmentActivity implements OnMapReadyCallback,
        GoogleApiClient.ConnectionCallbacks,
        GoogleApiClient.OnConnectionFailedListener,
        com.google.android.gms.location.LocationListener {
    private GoogleMap mMap;
    GoogleApiClient googleApiClient;
    Location lastLocation;
    LocationRequest locationRequest;
    private Button LogoutDriverButton, SettingsDriverButton;
    private FirebaseAuth mAuth;
    private String currentUser;
    private Boolean currentLogoutDriverStatus;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_map_driver);
        mAuth = FirebaseAuth.getInstance();
        currentUser = mAuth.getCurrentUser();
        LogoutDriverButton = (Button)findViewById(R.id.driver_loguot_button);
        SettingsDriverButton = (Button)findViewById(R.id.driver_setting_button);
        SupportMapFragment mapFragment = (SupportMapFragment) getSupportFragmentManager()
                .findFragmentById(R.id.map);
        mapFragment.getMapAsync(this);
        LogoutDriverButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                currentLogoutDriverStatus = true;
                mAuth.signOut();
                LogoutDriver();
                DisconnectDriver();
            }
        });
    }
    private void LogoutDriver() {
        Intent welcomeIntent = new Intent(MapDriverActivity.this, WcomeActivity2.class);
        startActivity(welcomeIntent);
        finish();
    }

    @Override
    public void onMapReady(GoogleMap googleMap) {
        mMap = googleMap;
        buildGoogleApiClient();
        if (ActivityCompat.checkSelfPermission(this, Manifest.permission.ACCESS_FINE_LOCATION) != PackageManager.PERMISSION_GRANTED && ActivityCompat.checkSelfPermission(this, Manifest.permission.ACCESS_COARSE_LOCATION) != PackageManager.PERMISSION_GRANTED) {
            return;
        }
        mMap.setMyLocationEnabled(true);
    }
    @Override
    public void onConnected(@Nullable Bundle bundle) {
        locationRequest = new LocationRequest();
        locationRequest.setInterval(1000);
        locationRequest.setFastestInterval(1000);
        locationRequest.setPriority(locationRequest.PRIORITY_HIGH_ACCURACY);
        if (ActivityCompat.checkSelfPermission(this, Manifest.permission.ACCESS_FINE_LOCATION) != PackageManager.PERMISSION_GRANTED && ActivityCompat.checkSelfPermission(this, Manifest.permission.ACCESS_COARSE_LOCATION) != PackageManager.PERMISSION_GRANTED) {
            return;
        }
        LocationServices.FusedLocationApi.requestLocationUpdates(googleApiClient, locationRequest, this);
    }
    @Override
    public void onConnectionSuspended(int i) {
    }
    @Override
    public void onConnectionFailed(@NonNull ConnectionResult connectionResult) {
    }
    @Override
    public void onLocationChanged( Location location) {
        lastLocation = location;
        LatLng latlng = new LatLng(location.getLatitude(), location.getLongitude());
        mMap.moveCamera(CameraUpdateFactory.newLatLng(latlng));
        mMap.animateCamera(CameraUpdateFactory.zoomTo(12));
        String userID = FirebaseAuth.getInstance().getCurrentUser().getUid();
        DatabaseReference DriverAvalablityRef = FirebaseDatabase.getInstance().getReference().child("Driver Available");
        GeoFire geoFire = new GeoFire(DriverAvalablityRef);
        geoFire.setLocation(userID, new GeoLocation(location.getLatitude(), location.getLongitude()));
    }
    protected synchronized void buildGoogleApiClient()
    {
        googleApiClient = new GoogleApiClient.Builder(this)
                .addConnectionCallbacks(this)
                .addOnConnectionFailedListener(this)
                .addApi(LocationServices.API)
                .build();

        googleApiClient.connect();
    }
    @Override
    protected void onStop() {
        super.onStop();
        if (!currentLogoutDriverStatus) {
            DisconnectDriver();
        }

    }

    private void DisconnectDriver() {
        String userID = FirebaseAuth.getInstance().getCurrentUser().getUid();
        DatabaseReference DriverAvalablityRef = FirebaseDatabase.getInstance().getReference().child("Driver Available");

        GeoFire geoFire = new GeoFire(DriverAvalablityRef);
        geoFire.removeLocation(userID);
    }

}