Android : Error while using ProgressDailog using in AsyncTask

I created Application on parsing XML data.I want to show loading progress dialog. I created two class on ListActivity(MainClass) and other is Download(Which execute in background using Asyntask).

I using Following code.

public class ListActivity extends Activity implements AsyncResponse {

    public ProgressDialog progressDialog;

    @Override
    public void onCreate(Bundle savedInstanceState) {

        // TODO Auto-generated method stub
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        progressDialog = new ProgressDialog(this);
        String url = "http://www.moneycontrol.com/rss/MCtopnews.xml";
        Download download = new Download();
        download.delegate = this;
        download.execute(url);

    }

    /*
     * After background task of download and parsing xml ArrayList received from
     * background task and send it to arrayAdapterzz
     */
    @Override
    public void processFinish(ArrayList<NewsItem> listArrayList) {
        ListView newsListView;

        newsListView = (ListView) findViewById(R.id.listView1);
        NewsListAdapter newsListAdapter = new NewsListAdapter(
                ListActivity.this, 0, listArrayList);
        newsListView.setAdapter(newsListAdapter);

    }
}


public class Download extends AsyncTask<String, Integer, ArrayList<NewsItem>> {
    public AsyncResponse delegate;
    private InputStream mInStream;
    private ArrayList<NewsItem> mNewsList;
    ListActivity la = new ListActivity();

    @Override
    protected void onPreExecute() {
        // TODO Auto-generated method stub
        super.onPreExecute();
        la.progressDialog.setMessage("Loading");
        la.progressDialog.setProgressStyle(ProgressDialog.STYLE_SPINNER);
        la.progressDialog.setProgress(0);
        la.progressDialog.show();
    }

    @Override
    protected ArrayList<NewsItem> doInBackground(String... params) {
...
..

I am getting this erro

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.rss/com.parse.ui.ListActivity}: java.lang.NullPointerException