Android : Button not working in Android AudioManager

Good day all, I'm new to android development and I'm trying to make a simple Audio Manager where the MainActivity class will handle button listener event and Listeners class will have silent(), vibrate(), ring() methods. So far app is launching but whenever I click on vibrate button app gets Force Close. I think there is something wrong in my code but I can't figure it out since I'm new.

MainActivity.java

public class MainActivity extends Activity {
private Button home, pocket, silent;
private TextView tv;
private Listeners myAudManHolder;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    AudioManager am = (AudioManager) getSystemService(Context.AUDIO_SERVICE);

    home = (Button) findViewById(R.id.home);
    pocket = (Button) findViewById(R.id.pocket);
    silent = (Button) findViewById(R.id.silent);
    tv = (TextView) findViewById(R.id.tv);

    myAudManHolder = new Listeners();

    pocket.setOnClickListener(new OnClickListener(){

        @Override
        public void onClick(View v) {
            myAudManHolder.vibrate();
        }

    });
}

Listeners.java

public class Listeners {
AudioManager myAudMan;

public void vibrate() {
    myAudMan.setRingerMode(AudioManager.RINGER_MODE_VIBRATE);
   }
}

LogCat Logs

10-28 20:57:51.381: W/dalvikvm(1232): threadid=1: thread exiting with uncaught exception (group=0x40015560)
10-28 20:57:51.401: E/AndroidRuntime(1232): FATAL EXCEPTION: main
10-28 20:57:51.401: E/AndroidRuntime(1232): java.lang.NullPointerException
10-28 20:57:51.401: E/AndroidRuntime(1232):     at edu.shihank.audiomanager.Listeners.vibrate(Listeners.java:13)
10-28 20:57:51.401: E/AndroidRuntime(1232):     at edu.shihank.audiomanager.MainActivity$1.onClick(MainActivity.java:39)

What is wrong in my vibrate() method and its OnClickListener event? Please I need this solution today and your help would be really appreciable. Tnx.