Android : how to stop a running async task?

I am running a async task in android to ping t a particular URL but the problem is when the url is not valid or not reachable an exception occurs as sockettimeoutexception so on that exception i want to stop the running task. iagve tried using Cancel() method but that causes App crash.

I am using the following code.

private class UrlDataProvider3 extends AsyncTask<String, Void, String>
{

    String ret="";
    int checkStatus;
    Boolean exception=false;
    @Override
    protected String doInBackground(String... url) 
    {

        HttpURLConnection con = null;

        try 
          { 

            Log.i("RAE", "urlData"+url[0]);
            HttpURLConnection.setFollowRedirects(true);
             con = (HttpURLConnection) new URL(url[0]).openConnection();
                 con.setRequestMethod("POST");
              con.setConnectTimeout(20000);




          }


        catch (IOException e)
          {


            if(e.toString().contains("java.net.SocketTimeoutException:"))
            {



                return null;

            }



          }



    return ret;
    }
    @Override
    protected void onPostExecute(String result) {
        // TODO Auto-generated method stub
        super.onPostExecute(result);
        Log.i("RAE"," Asyc finished");



}