Android : Null Pointer Exception while retrieving json object using php?

android code:

protected String doInBackground(String... params) {
        // TODO Auto-generated method stub

        try {
            SharedPreferences prefs = getActivity().getSharedPreferences("Login_Prefs", 0);

            username = prefs.getString("username", "defValue");

            String url = "http://www.iloveexpressions.com/eh/viewFriendList.php";

            nameValuePairs = new ArrayList<NameValuePair>(1);

            Log.d("Parameters ", username);

            nameValuePairs.add(new BasicNameValuePair("username", username.trim()));

            Log.d("name value pairs", nameValuePairs.toString());

            JSONObject json = jsonParser.makeHttpRequest(url, "POST",
                    nameValuePairs);

            Log.d("Printing after json", json.toString());

            success = json.getString("success");

            Log.d("Success : ", success);

        } catch (Exception e) {
            Log.d("Error : ", e.toString());
        }

php code:

  <?php

require 'connection.php';

   $response = array();

if(isset($_POST["username"]))
 {
$username = $_POST["username"];

$response["success"] = 1;

echo json_encode($response);
 }
else
 {
$response["success"] = 0;

echo json_encode($response);
}

?>

This is the error i get in log :

Parameters(30889): aaa

name value pairs(30889): [username=aaa]

Error :(30889): java.lang.NullPointerException

How do I solve this error ? I am using the same method to send and retrieve data using php on android and it works fine ? I am returning success if username is sent to php file correctly

I am using the doInBackground process in a fragment and calling the execute() method in the onCreateView() method is this possible ?