Android : java.lang.NullPointerException when I'm trying to make button invisible

This method works fine.

public void onClick(View view) { 
btn = (Button) findViewById(R.id.trytogetin);
progr = (ProgressBar) findViewById(R.id.progressBar);
btn.setVisibility(View.INVISIBLE);
progr.setVisibility(View.VISIBLE);
EditText login = (EditText) findViewById(R.id.loginfld);
EditText passw = (EditText) findViewById(R.id.passfld);
String logincmd = "CheckLogin*" + login.getText() + "*" + passw.getText() + "*";
ss.senddata(logincmd, 1);
}

In this method java.lang.NullPointerException appears (on btn.setVisibility(View.VISIBLE);

    public void geturdata(String answer) {
    if (answer != null)
    {
        System.out.println("true");
         btn.setVisibility(View.VISIBLE);
    }
    else
    {
        System.out.println("false");
    }
}

Please tell me how can I call button in this method? Also I can't use StartActivity(intent) in this method.(same error). Both metods placed in one activity.