Problem with Asyntask-PHP

My PHP

<?php
require "dbconfig.php";

$con=mysqli_connect($servername,$username,$password,$dbname) or die('Unable to connect');
if(mysqli_connect_error($con))
{
  echo "Failed to Connect to Database ".mysqli_connect_error();
}
$name=$_POST['Query'];
$sql="SELECT name_user,SUM(cash_car) FROM usuario u 
INNER JOIN 
carro c ON u.id_user=c.id_user 
WHERE u.name_user='elias';";
$query=mysqli_query($con,$sql);
if($query)
{
    while($row=mysqli_fetch_array($query))
  {
    $data[]=$row;
  }
    print(json_encode($data));
}else
{
  echo('Not Found ');
}
mysqli_close($con);

I get the following from the file PHP

[{"0":"elias","name_user":"elias","1":"10","SUM(cash_car)":"10"}]

And I just wanted to show the total of the SUM function which is 10 in this case, since the other field (user_name) is only to filter the result.

But executing the query using asyntask the app stops working.

 private class AsyncRetrieve extends AsyncTask<String, String, String> {
        ProgressDialog pdLoading = new ProgressDialog(ganancias_totales.this);
        HttpURLConnection conn;
        URL url = null;

        @Override
        protected void onPreExecute() {
            super.onPreExecute();

            pdLoading.setMessage("\tConsultando...");
            pdLoading.setCancelable(false);
            pdLoading.show();

        }


        @Override
        protected String doInBackground(String... params) {
            try {

                url = new URL("http://bdauditorio.esy.es/total_carros/ver_total_elias.php");

            } catch (MalformedURLException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
                return e.toString();
            }
            try {


                conn = (HttpURLConnection) url.openConnection();
                conn.setReadTimeout(READ_TIMEOUT);
                conn.setConnectTimeout(CONNECTION_TIMEOUT);
                conn.setRequestMethod("GET");


                conn.setDoOutput(true);

            } catch (IOException e1) {
                // TODO Auto-generated catch block
                e1.printStackTrace();
                return e1.toString();
            }

            try {

                int response_code = conn.getResponseCode();


                if (response_code == HttpURLConnection.HTTP_OK) {


                    InputStream input = conn.getInputStream();
                    BufferedReader reader = new BufferedReader(new InputStreamReader(input));
                    StringBuilder result = new StringBuilder();
                    String line;

                    while ((line = reader.readLine()) != null) {
                        result.append(line);
                    }


                    return (result.toString());

                } else {

                    return ("unsuccessful");
                }

            } catch (IOException e) {
                e.printStackTrace();
                return "exception";
            } finally {
                conn.disconnect();
            }


        }


        @Override
        protected void onPostExecute(String result) {

            pdLoading.dismiss();
            if (result.equalsIgnoreCase("exception") || result.equalsIgnoreCase("unsuccessful")) {
                final AlertDialog.Builder alertaDeError = new AlertDialog.Builder(ganancias_totales.this);
                alertaDeError.setTitle("Error");
                alertaDeError.setMessage("Ups, no se han podido cargar las cuentas. Intentelo de nuevo.");
                alertaDeError.setPositiveButton("Aceptar", new DialogInterface.OnClickListener() {
                    @Override
                    public void onClick(DialogInterface dialog, int which) {
                    }
                });
                alertaDeError.create();
                alertaDeError.show();
            } else {
                //Existen Datos
                List<String> preguntas = new ArrayList<String>();
                JSONArray jsonArray = null;
                try {
                    jsonArray = new JSONArray(result);

                    for (int i = 0; i < jsonArray.length(); i++) {
                        JSONObject preguntaDatos = null;
                        try {
                            preguntaDatos = jsonArray.getJSONObject(i);
                        } catch (JSONException e) {
                            e.printStackTrace();
                        }

                        try {
                            assert preguntaDatos != null;
                            pregrespcomment =" Cuenta" + "\n" +"> Total ganacias: "+ preguntaDatos.getString("cash_car");

                        } catch (JSONException e) {
                            e.printStackTrace();
                        }
                        preguntas.add(pregrespcomment);

                    }

                    //crear el Adapter.
                    ArrayAdapter<String> adapter = new ArrayAdapter<String>(ganancias_totales.this, android.R.layout.simple_list_item_1, preguntas);
                    mostrarr.setAdapter(adapter);
                   // mostrarr.getAdapter().getCount();
                    //Toast.makeText(getApplicationContext(), "Total de cuentas: " + mostrarr.getAdapter().getCount() , Toast.LENGTH_LONG).show();

                } catch (Exception e) {
                    e.printStackTrace();

                    final AlertDialog.Builder alertaDeError = new AlertDialog.Builder(ganancias_totales.this);
                    alertaDeError.setTitle("Error");
                    alertaDeError.setMessage("Ups, no existen cuentas para mostrar. Intentelo de nuevo.");
                    alertaDeError.setPositiveButton("Aceptar", new DialogInterface.OnClickListener() {
                        @Override
                        public void onClick(DialogInterface dialog, int which) {
                        }
                    });
                    alertaDeError.create();
                    alertaDeError.show();
                }
            }
        }


    }