Async android json parser android- null pointer exception

From my last post, got t know that, i need to use async task for json parsing from url, have done the same and attached below,

public class ReadJson extends ListActivity {
private static String url = "http://docs.blackberry.com/sampledata.json";

private static final String TAG_VTYPE = "vehicleType";
private static final String TAG_VCOLOR = "vehicleColor";
private static final String TAG_FUEL = "fuel";
private static final String TAG_TREAD = "treadType";

ArrayList> jsonlist = new ArrayList>();

ListView lv ;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_read_json);
    new ProgressTask(ReadJson.this).execute();
}
private class ProgressTask extends AsyncTask {
    private ProgressDialog dialog;
    // private List messages;
    public ProgressTask(ListActivity activity) {
        context = activity;
        dialog = new ProgressDialog(context);
    }
    /** progress dialog to show user that the backup is processing. */
    /** application context. */
    private Context context;
    protected void onPreExecute() {
        this.dialog.setMessage("Progress start");
        this.dialog.show();
    }
    @Override
    protected void onPostExecute(final Boolean success) {
        if (dialog.isShowing()) {
            dialog.dismiss();
        }
        ListAdapter adapter = new SimpleAdapter(context, jsonlist,
                R.layout.list_item, new String[] { TAG_VTYPE, TAG_VCOLOR,
                TAG_FUEL, TAG_TREAD }, new int[] {
                R.id.vehicleType, R.id.vehicleColor, R.id.fuel,
                R.id.treadType });
        setListAdapter(adapter);
        // selecting single ListView item
        lv = getListView();
    }
    protected Boolean doInBackground(final String... args) {
        JSONParser jParser = new JSONParser();
        JSONArray json = jParser.getJSONFromUrl(url);
        for (int i = 0; i < json.length(); i++) {
            try {
                JSONObject c = json.getJSONObject(i);
                String vtype = c.getString(TAG_VTYPE);
                String vcolor = c.getString(TAG_VCOLOR);
                String vfuel = c.getString(TAG_FUEL);
                String vtread = c.getString(TAG_TREAD);
                HashMap map = new HashMap();
                map.put(TAG_VTYPE, vtype);
                map.put(TAG_VCOLOR, vcolor);
                map.put(TAG_FUEL, vfuel);
                map.put(TAG_TREAD, vtread);
                jsonlist.add(map);
            } catch (JSONException e) {
                e.printStackTrace();
            }
        }
        return null;
    }
}

}

When i execute this, i get null pointer exception for error executing in asyc background in the line, for (int i = 0; i < json.length(); i++), tried several things but not working, any help will be gr8ly appreciated!!

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