Bad notification for startForeground: java.lang.RuntimeException

i'm developing location tracking application by using firebase, the app get the device location, when we don't want to track the location we click on the notification of the app to desable it, i got an error like this

android.app.RemoteServiceException: Bad notification for startForeground: java.lang.RuntimeException: invalid channel for service notification: Notification(channel=null pri=0 contentView=null vibrate=null sound=null defaults=0x0 flags=0x42 color=0x00000000 vis=PRIVATE)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1768)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:164)
        at android.app.ActivityThread.main(ActivityThread.java:6494)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)

can anyone tell me whats the problem and how to solve this.

trackerservice.java

public class TrackerService extends Service {

    private static final String TAG = TrackerService.class.getSimpleName();

    @Override
    public IBinder onBind(Intent intent) {return null;}

    @Override
    public void onCreate() {
        super.onCreate();
        buildNotification();
        loginToFirebase();
    }

    private void buildNotification() {
        String stop = "stop";
        registerReceiver(stopReceiver, new IntentFilter(stop));
        PendingIntent broadcastIntent = PendingIntent.getBroadcast(
                this, 0, new Intent(stop), PendingIntent.FLAG_UPDATE_CURRENT);
        // Create the persistent notification
        NotificationCompat.Builder builder = new NotificationCompat.Builder(this)
                .setContentTitle(getString(R.string.app_name))
                .setContentText(getString(R.string.notification_text))
                .setOngoing(true)
                .setContentIntent(broadcastIntent)
                .setSmallIcon(R.drawable.ic_tracker);
        startForeground(1, builder.build());
    }

    protected BroadcastReceiver stopReceiver = new BroadcastReceiver() {
        @Override
        public void onReceive(Context context, Intent intent) {
            Log.d(TAG, "received stop broadcast");
            // Stop the service when the notification is tapped
            unregisterReceiver(stopReceiver);
            stopSelf();
        }
    };

    private void loginToFirebase() {
            // Authenticate with Firebase, and request location updates
            String email = getString(R.string.firebase_email);
            String password = getString(R.string.firebase_password);
            FirebaseAuth.getInstance().signInWithEmailAndPassword(
                    email, password).addOnCompleteListener(new OnCompleteListener<AuthResult>(){
                @Override
                public void onComplete(Task<AuthResult> task) {
                    if (task.isSuccessful()) {
                        Log.d(TAG, "firebase auth success");
                        requestLocationUpdates();
                    } else {
                        Log.d(TAG, "firebase auth failed");
                    }
                }
            });
        }

    private void requestLocationUpdates() {
            LocationRequest request = new LocationRequest();
            request.setInterval(10000);
            request.setFastestInterval(5000);
            request.setPriority(LocationRequest.PRIORITY_HIGH_ACCURACY);
            FusedLocationProviderClient client = LocationServices.getFusedLocationProviderClient(this);
            final String path = getString(R.string.firebase_path) + "/" + getString(R.string.transport_id);
            int permission = ContextCompat.checkSelfPermission(this,
                    Manifest.permission.ACCESS_FINE_LOCATION);
            if (permission == PackageManager.PERMISSION_GRANTED) {
                // Request location updates and when an update is
                // received, store the location in Firebase
                client.requestLocationUpdates(request, new LocationCallback() {
                    @Override
                    public void onLocationResult(LocationResult locationResult) {
                        DatabaseReference ref = FirebaseDatabase.getInstance().getReference(path);
                        Location location = locationResult.getLastLocation();
                        if (location != null) {
                            Log.d(TAG, "location update " + location);
                            ref.setValue(location);
                        }
                    }
                }, null);
            }
        }
    }

MainActivity.java

public class MainActivity extends Activity {

    private static final int PERMISSIONS_REQUEST = 1;

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

        // Check GPS is enabled
        LocationManager lm = (LocationManager) getSystemService(LOCATION_SERVICE);
        if (!lm.isProviderEnabled(LocationManager.GPS_PROVIDER)) {
            Toast.makeText(this, "Please enable location services", Toast.LENGTH_SHORT).show();
            finish();
        }

        // Check location permission is granted - if it is, start
        // the service, otherwise request the permission
        int permission = ContextCompat.checkSelfPermission(this,
                Manifest.permission.ACCESS_FINE_LOCATION);
        if (permission == PackageManager.PERMISSION_GRANTED) {
            startTrackerService();
        } else {
            ActivityCompat.requestPermissions(this,
                    new String[]{Manifest.permission.ACCESS_FINE_LOCATION},
                    PERMISSIONS_REQUEST);
        }
    }

    private void startTrackerService() {
        startService(new Intent(this, TrackerService.class));
        finish();
    }

    @Override
    public void onRequestPermissionsResult(int requestCode, String[] permissions, int[]
            grantResults) {
        if (requestCode == PERMISSIONS_REQUEST && grantResults.length == 1
                && grantResults[0] == PackageManager.PERMISSION_GRANTED) {
            // Start the service when the permission is granted
            startTrackerService();
        } else {
            finish();
        }
    }
}