how to use doInBackground in android for show error msg

Hello i am using AsyncTask for webservice calling. Its working fine when i enter correct username and password, when i enter wrong then its how msg wrong username and password but in my code in doInBackground area its throw expction or application crass
private class LoginTask extends AsyncTask {

    private final ProgressDialog dialog = new ProgressDialog(login.this); 

    protected void onPreExecute() { 


            this.dialog.setMessage("Logging in..."); 
            this.dialog.show(); 
    } 


    protected Void doInBackground(final Void... unused) { 
        String auth=calllogin(Username,Passowrd); 
        cls_constant.userid=auth;
        if(auth.equals("0"))
        {


        TextView errorTextview=(TextView)findViewById(R.id.tv_error);
        errorTextview.setText("Username & Password incorrect.");
        }
        else {

        Intent intnt=new Intent(getBaseContext(), aftersignin.class);
        startActivity(intnt);
        }
        return null; // don't interact with the ui! 

    } 


    protected void onPostExecute(Void result)  
    { 

            if (this.dialog.isShowing()) 
            { 
            this.dialog.dismiss(); 
            }



     } 

i want ot show error mgs like this but i dont know whats i m doing wrong
or how to use onPostExecute for show msg
thanks

Answer

You cant update TextView on a different thread (since Asyn does job in a new thread) since it belongs to UI thread. So fix your doInBackground as below:

protected Void doInBackground(final Void... unused) { 
    String auth=calllogin(Username,Passowrd); 
    cls_constant.userid=auth;
    if(auth.equals("0"))
    {
      YourActivityClass.this.runOnUiThread(new Runnable() {
        @Override
        public void run() {
            TextView errorTextview=(TextView)findViewById(R.id.tv_error);

                errorTextview.setText("Username & Password incorrect.");
        }
      });
    }
    else {

    Intent intnt=new Intent(getBaseContext(), aftersignin.class);
    startActivity(intnt);
    }
    return null; // don't interact with the ui! 

}