How to search for information in the database, with the application running in the background (in service)?

I'm new to the Android branch, and I'm making an application that generates a notification when it gets close to places registered in the database, but it's giving an error:

  

E / AndroidRuntime: FATAL EXCEPTION: main       Process: splashdev.devmedia.edu.br.location_rone, PID: 3938       java.lang.RuntimeException: Unable to start service splashdev.devmedia.edu.br.local_rone.GCMService@6694939 with Intent {cmp = splashdev.devmedia.edu.br.location_rone / .GCMService}: android.view.WindowManager $ BadTokenException: Unable to add window - token null is not for an application

My code:

public class GCMService extends Service{

    public static final String TAG = "GCMService";
    LocationRequest mLocationRequest;
    FusedLocationProviderClient mFusedLocationClient;
    LocationCallback mLocationCallback;
    int raio = 100;
    private NotificationManagerCompat notificationManager;


    ArrayList<Double> loc = new ArrayList<Double>();
    ArrayList<String> locaisnomes = new ArrayList<>();
    public static ArrayList<String> listaEmpresasNotificadas = new ArrayList<>();

    @Override
    public IBinder onBind(Intent arg0) {
        return null;
    }

    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        Log.e(TAG, "onStartCommand");
        notificationManager = NotificationManagerCompat.from(this);

        BuscaPontos();

        mFusedLocationClient = LocationServices.getFusedLocationProviderClient(this);
        initLocationRequest();
        mLocationCallback = new LocationCallback() {
            @Override
            public void onLocationResult(LocationResult locationResult) {
                String s= "";
                if (locationResult == null) Toast.makeText(GCMService.this, "\nLocationResult == null", Toast.LENGTH_LONG).show();

                else {
                    for (Location location : locationResult.getLocations()) {

                        for(int i=0, j=0; i<loc.size();i+=2,j++) {
                            double dist = calcDistancia(location.getLatitude(), location.getLongitude(), loc.get(i), loc.get(i + 1));
                            dist = dist * 1000;

                            if (dist < raio) {
                                listaEmpresasNotificadas.add(locaisnomes.get(j));

                            }
                            else{}
                            if (verificarNotificacao(locaisnomes.get(j))==false){

                                Intent intent = new Intent(getApplicationContext(), MainActivity.class);
                                intent.putExtra("Location", location);
                                intent.putExtra("Parceiro", locaisnomes.get(j));

                                PendingIntent contentIntent = PendingIntent.getActivity(getApplicationContext(), 0, intent, PendingIntent.FLAG_UPDATE_CURRENT);
                                showNotification(getApplicationContext(), "Há descontos por perto!", locaisnomes.get(j) + " é uma empresa parceira e está próximo de você", contentIntent);

                                break;

                            }
                        }
                    }
                }
            }
        };
        startLocationUpdates();
        return START_STICKY;
    }

    private void initLocationRequest(){
        mLocationRequest = new LocationRequest();
        mLocationRequest.setInterval(2000);
        mLocationRequest.setFastestInterval(1000);
        mLocationRequest.setPriority(LocationRequest.PRIORITY_HIGH_ACCURACY);
    }
    private void startLocationUpdates() {
        if(Build.VERSION.SDK_INT>= Build.VERSION_CODES.M){
            //Verifica se já tem permissão
            //Se não tem, pede a permissão
            if(ContextCompat.checkSelfPermission(this,
                    Manifest.permission.ACCESS_FINE_LOCATION)==
                    PackageManager.PERMISSION_GRANTED){
            }
        }
        else { }

        mFusedLocationClient.requestLocationUpdates(mLocationRequest, mLocationCallback,null /* Looper */);
    }


    private double calcDistancia(double lat_inicial, double long_inicial, double lat_final, double long_final) {

        double d2r = 0.017453292519943295769236;

        double dlong = (long_final - long_inicial) * d2r;
        double dlat = (lat_final - lat_inicial) * d2r;

        double temp_sin = Math.sin(dlat/2.0);
        double temp_cos = Math.cos(lat_inicial * d2r);
        double temp_sin2 = Math.sin(dlong/2.0);

        double a = (temp_sin * temp_sin) + (temp_cos * temp_cos) * (temp_sin2 * temp_sin2);
        double c = 2.0 * Math.atan2(Math.sqrt(a), Math.sqrt(1.0 - a));

        return 6368.1 * c;
    }

    public void BuscaPontos(){
        HashMap postData = new HashMap();

        PostResponseAsyncTask task1 = new PostResponseAsyncTask(GCMService.this, postData,
                new AsyncResponse() {
                    @Override
                    public void processFinish(String s) {
                        String x = "";
                        try {
                            JSONArray jsonArray = new JSONArray(s);
                            for(int i=0; i<jsonArray.length();i++){
                                JSONObject aux = (JSONObject) jsonArray.get(i);
                                String lat = aux.getString("latitude");
                                String longi = aux.getString("longitude");
                                String nloja = aux.getString("nomeloja");
                                x = x + "\n" + "lat:" + lat + "    long:" + longi + "   nome da loja" + nloja;

                                double latitude = Double.parseDouble(lat);
                                double longitude = Double.parseDouble(longi);

                                loc.add(latitude);
                                loc.add(longitude);
                                locaisnomes.add(nloja);
                            }
                        } catch (JSONException e) {
                            e.printStackTrace();
                        }

                    }
                });
        //ip = Sessao.getIp();
        task1.execute("http://192.168.2.113/phpitgirl/buscapontos.php");

    }

    public static boolean verificarNotificacao(String empresa) {
        for (int i = 0; i < listaEmpresasNotificadas.size(); i++) {
            if (listaEmpresasNotificadas.get(i).equals(empresa)) {
                return true;
            }
        };

        return false;
    }

    public void showNotification(Context context, String title, String body, PendingIntent intent) {
        NotificationManager notificationManager = (NotificationManager) context.getSystemService(Context.NOTIFICATION_SERVICE);

        int notificationId = 2;
        String channelId = "notificacoes-01";
        String channelName = "Notificacoes";
        int importance = NotificationManager.IMPORTANCE_HIGH;

        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
            NotificationChannel mChannel = new NotificationChannel(
                    channelId, channelName, importance);
            notificationManager.createNotificationChannel(mChannel);
        }


        NotificationCompat.Builder mBuilder = new NotificationCompat.Builder(context, channelId)
                .setSmallIcon(R.drawable.ic_launcher_background)
                .setContentTitle(title)
                .setContentText(body)
                .setContentIntent(intent)
                .setDefaults(Notification.DEFAULT_SOUND)
                .setAutoCancel(true);

        notificationManager.notify(notificationId, mBuilder.build());
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
    }
}

MainActivity class:

public class MainActivity extends AppCompatActivity {

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        startService(new Intent(getBaseContext(), GCMService.class));
    }

}

BootCompletedIntentReceiver class:

public class BootCompletedIntentReceiver extends BroadcastReceiver {

    @Override
    public void onReceive(Context context, Intent intent) {
        if ("android.intent.action.BOOT_COMPLETED".equals(intent.getAction())) {
            Intent pushIntent = new Intent(context, GCMService.class);
            context.startService(pushIntent);
        }
    }
}