FriendPickerFragment Facebook Android SDK not showing anything

Question

i don't know what's wrong with my code. As long as i know my code is right, but i still got java.lang.null.pointer in

friendPickerFragment.loadData(false);

i already defined friendPickerFragment like this but still, i got error

friendPickerFragment = (FriendPickerFragment) fragmentManager.findFragmentById(R.id.friend_picker_fragment);

Here is my whole code and i think it's the same as facebook sdk sample friendpicker

public class PickFriends extends FragmentActivity {

FriendPickerFragment friendPickerFragment;

public static void populateParameters(Intent intent, String userId, boolean multiSelect, boolean showTitleBar){
    intent.putExtra(FriendPickerFragment.USER_ID_BUNDLE_KEY,userId);
    intent.putExtra(FriendPickerFragment.MULTI_SELECT_BUNDLE_KEY, multiSelect);
    intent.putExtra(FriendPickerFragment.SHOW_TITLE_BAR_BUNDLE_KEY,showTitleBar);
}

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_pick_friends);

    FragmentManager fragmentManager = getSupportFragmentManager();

    if(savedInstanceState == null){
        final Bundle args = getIntent().getExtras();
        friendPickerFragment = new FriendPickerFragment(args);
        fragmentManager.beginTransaction()
                .add(R.id.friend_picker_fragment, friendPickerFragment);
        //friendPickerFragment = (FriendPickerFragment) fragmentManager.findFragmentById(R.id.friend_picker_fragment);
    } else {
        friendPickerFragment = (FriendPickerFragment) fragmentManager.findFragmentById(R.id.friend_picker_fragment);
    }

    friendPickerFragment.setOnErrorListener(new PickerFragment.OnErrorListener() {
        @Override
        public void onError(PickerFragment<?> fragment, FacebookException error) {
            PickFriends.this.onError(error);
        }
    });

    friendPickerFragment.setOnDoneButtonClickedListener(new PickerFragment.OnDoneButtonClickedListener() {
        @Override
        public void onDoneButtonClicked(PickerFragment<?> fragment) {
            SelectFriend application = (SelectFriend) getApplication();
            application.setSelectedUsers(friendPickerFragment.getSelection());

            setResult(RESULT_OK,null);
            finish();
        }
    });
}

private void onError(Exception error){
    String text = "Error Exception";
    Toast toast = Toast.makeText(this,text,Toast.LENGTH_SHORT);
    toast.show();
}

@Override
protected void onStart(){
    super.onStart();
    friendPickerFragment.loadData(false);
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.menu_pick_friends, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();

    //noinspection SimplifiableIfStatement
    if (id == R.id.action_settings) {
        return true;
    }

    return super.onOptionsItemSelected(item);
}

}

Please help me. Thank you