Android Relating users in parse.com. I can't select a friend from the list

public class EditFriendsActivity extends ListActivity {
    public static final String TAG = EditFriendsActivity.class.getSimpleName();
    protected List<ParseUser> mUsers;
    protected ParseRelation<ParseUser> friendRelation;
    protected ParseUser currentUser;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        requestWindowFeature(Window.FEATURE_INDETERMINATE_PROGRESS);
        setContentView(R.layout.activity_edit_friends);
        getListView().setChoiceMode(ListView.CHOICE_MODE_MULTIPLE);

        if (savedInstanceState == null) {
            getFragmentManager().beginTransaction()
                    .add(R.id.container, new PlaceholderFragment()).commit();
        }
        //getListView().setChoiceMode(ListView.CHOICE_MODE_MULTIPLE);
    }

    @Override
    protected void onResume() {
        // TODO Auto-generated method stub
        super.onResume();

        currentUser = ParseUser.getCurrentUser();
        friendRelation = currentUser.getRelation(ParseConstants.KEY_FRIEND_RELATION);

        setProgressBarIndeterminateVisibility(true);
        ParseQuery<ParseUser> query = ParseUser.getQuery();
        query.orderByAscending(ParseConstants.KEY_USERNAME);
        query.setLimit(1000);
        query.findInBackground(new FindCallback<ParseUser>() {

            @Override
            public void done(List<ParseUser> users, ParseException e) {
                setProgressBarIndeterminateVisibility(false);
                // TODO Auto-generated method stub
                if(e==null){
                    mUsers = users;
                    String[] usernames = new String[mUsers.size()];
                    int i = 0;
                    for(ParseUser us :mUsers){
                        usernames[i]=us.getUsername();
                        i++;
                    }
                    ArrayAdapter<String> adapter = new ArrayAdapter<String>(
                            EditFriendsActivity.this,
                            android.R.layout.simple_list_item_checked,
                            usernames);
                    setListAdapter(adapter);
                    addFriendCheckmarks();

                } else {
                    Log.e(TAG, e.getMessage());
                    AlertDialog.Builder builder = new AlertDialog.Builder(EditFriendsActivity.this);
                    builder.setMessage(e.getMessage())
                            .setTitle(R.string.error_title)
                            .setPositiveButton(android.R.string.ok, null);
                    AlertDialog dialog = builder.create();
                    dialog.show();
                }
            }
        });
    }

    @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();
        if (id == R.id.action_settings) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }

    /**
     * A placeholder fragment containing a simple view.
     */
    public static class PlaceholderFragment extends Fragment {
        public PlaceholderFragment() {
        }

        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container,
                Bundle savedInstanceState) {
            View rootView = inflater.inflate(R.layout.activity_edit_friends,
                    container, false);
            return rootView;
        }
    }
@Override
protected void onListItemClick(ListView l, View v, int position, long id) {
    // TODO Auto-generated method stub
    super.onListItemClick(l, v, position, id);
    if(getListView().isItemChecked(position)){
        //add friend
        friendRelation.add(mUsers.get(position));

    } else {
        //remove friend
        friendRelation.remove(mUsers.get(position));    
    }
    currentUser.saveInBackground(new SaveCallback() {

        @Override
        public void done(ParseException e) {
            // TODO Auto-generated method stub
            if(e!=null){
                Log.e(TAG, e.getMessage());
            }       
        }
    });
}
private void addFriendCheckmarks(){
    friendRelation.getQuery().findInBackground(new FindCallback<ParseUser>() {

        @Override
        public void done(List<ParseUser> friends, ParseException e) {
            // TODO Auto-generated method stub
            if(e==null){
                for(int i =0;i <mUsers.size();i++){
                    ParseUser usr = mUsers.get(i);
                    for(ParseUser fr : friends){
                        if(fr.getObjectId().equals(usr.getObjectId())){
                            getListView().setItemChecked(i,true);
                        }
                    }
                }
            }else{
                Log.e(TAG, e.getMessage());
            }
        }
    }); 
}
}