ارور هنگام اجرای برنامه اتصال به سرور

من برنامه ارتباط با سرور نوشتم ولی هنگام اجرا ارور میده.میشه راهنمایی بفرمایید :

public class MainActivity extends ListActivity {


    ProgressDialog pDialog;
    JSONParser jparser = new JSONParser();

    ArrayList<HashMap<String, String>> p;
    JSONArray a = null;

    final String url = "http://amarddev.yourgig.ir/travel.php";

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

        new travel().execute();

    }


    class travel extends AsyncTask<String, String, String> {

        @Override
        protected void onPreExecute() {
            super.onPreExecute();
            pDialog = new ProgressDialog(MainActivity.this);
            pDialog.setMessage("ارتباط با ستاد");
            pDialog.show();

        }

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

            List<NameValuePair> parms = new ArrayList<>();
            JSONObject json = jparser.makeHttpRequest(url, "GET", parms);
            try {
                int t = json.getInt("t");
                if (t == 1) {
                    a = json.getJSONArray("travel");
                    for (int i = 0; i < a.length(); i++) {
                        JSONObject c = a.getJSONObject(i);
                        String companyname = c.getString("companyname");
                        String cod = c.getString("cod");
                        String time = c.getString("time");
                        String date = c.getString("date");
                        String bign = c.getString("bign");
                        String stop = c.getString("stop");
                        String price = c.getString("price");

                        HashMap<String, String> map = new HashMap<String, String>();
                        map.put("companyname", companyname);
                        map.put("cod", cod);
                        map.put("time", time);
                        map.put("date", date);
                        map.put("bign", bign);
                        map.put("stop", stop);
                        map.put("price", price);

                        p.add(map);

                    }


                } else {
                  //  Toast.makeText(getApplicationContext(), "اطلاعاتی دریافت نشد", Toast.LENGTH_LONG).show();
                }

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


            return null;
        }

        @Override
        protected void onPostExecute(String s) {
            super.onPostExecute(s);
            pDialog.cancel();

            runOnUiThread(new Runnable() {
                @Override
                public void run() {

                    ListAdapter adapter = new SimpleAdapter(MainActivity.this, p, R.layout.list_item,
                            new String[]{"companyname", "cod", "bign", "stop", "time", "date", "price"},
                            new int[]{R.id.companyname, R.id.cod, R.id.bign, R.id.stop, R.id.time, R.id.date, R.id.price});
                    setListAdapter(adapter);
                }
            });


        }
    }
}

java.lang.RuntimeException: An error occured while executing doInBackground()