Android : getting NullPointerException while setting SharedPreferences

i'm trying to set the SharedPrefencesin my onPostExecute method but i get NullPointerException on my error log. i've tried to put it doInBackground but that block my result to be returned. So this is the code

My Task class

public class LoginTask extends AsyncTask<Void, Void, String> {

    private final String mEmail;
    private final String mPassword;

    private  SharedPreferences sharedpreferences;
    private final String MyPREFERENCES = "session" ;

    String url = ****

    LoginTask(String email, String password) {
        mEmail = email;
        mPassword = password;
    }

    @TargetApi(Build.VERSION_CODES.HONEYCOMB)
    @Override
    public String doInBackground(Void... params) {
       // TODO: attempt authentication against a network service.

        try {
            // Simulate network access.
            Thread.sleep(2000);
        } catch (InterruptedException e) {
            return "false";
        }

        ArrayList<BasicNameValuePair> nameValuePairs = new ArrayList<BasicNameValuePair>();
        nameValuePairs.add(new BasicNameValuePair("email", mEmail));
        nameValuePairs.add(new BasicNameValuePair("password", mPassword));

        //Prepare the Post query

        try {
            HttpClient clientHttp = new DefaultHttpClient();
            HttpPost httppost = new HttpPost(url);
            httppost.setEntity(new UrlEncodedFormEntity(nameValuePairs));
            HttpResponse response = clientHttp.execute(httppost);
            HttpEntity entity = response.getEntity();
            InputStream is = entity.getContent();
            BufferedReader reader = new BufferedReader(new InputStreamReader(is,"iso-8859-1"),8);
            StringBuilder sb = new StringBuilder();
            String line = reader.readLine();
            sb.append(line + "\n");
            is.close();

            String result = sb.toString();

            System.out.println(result);


            return result;



        }catch (Exception e){

            return null;
        }

    }

    @Override
    protected void onPostExecute(final String res) {
        try {
             jObj = new JSONObject(res);
        } catch (JSONException e) {
            e.printStackTrace();
        }
        try {
            if (jObj.getString("code").equals("1")) {

                SharedPreferences.Editor e = sharedpreferences.edit();
                e.putString(MyPREFERENCES,"active");
                e.commit();

                Intent myIntent = new Intent(getActivity(), HomesActivity.class);

                //Lançer l'activité
                startActivityForResult(myIntent, 0);

            } else {
                //password.setError(getString(R.string.error_incorrect_password));
                //password.requestFocus();
                AlertDialog.Builder builder = new AlertDialog.Builder(getActivity());
                builder.setTitle("Error !");
                builder.setMessage("The information entered is incorrect.\nPlease try again!")
                        .setCancelable(false)
                        .setPositiveButton("OK", new DialogInterface.OnClickListener() {
                            public void onClick(DialogInterface dialog, int id) {
                                dialog.cancel();
                            }
                        });
                AlertDialog alert = builder.create();
                alert.show();

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

    }

    @Override
    protected void onCancelled() {
        LoginTask = null;
        //showProgress(false);
    }
}

My error Log :

10-09 12:58:50.088    2878-2878/com.example.user.unchained E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: com.example.user.unchained, PID: 2878
java.lang.NullPointerException
        at com.example.user.unchained.EmailLoginActivity$LoginTask.onPostExecute(EmailLoginActivity.java:328)
        at com.example.user.unchained.EmailLoginActivity$LoginTask.onPostExecute(EmailLoginActivity.java:255)
        at android.os.AsyncTask.finish(AsyncTask.java:632)
        at android.os.AsyncTask.access$600(AsyncTask.java:177)
        at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:645)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:136)
        at android.app.ActivityThread.main(ActivityThread.java:5017)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:515)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
        at dalvik.system.NativeStart.main(Native Method)