GridView check image are not added when grid is tapped

public class SearchActivity extends Activity {

private ParseQueryAdapter<ParseUser> mainAdapter;

protected ParseRelation<ParseUser> mFriendsRelation;
protected ParseUser mCurrentUser;
protected GridView mGridView;

@InjectView(R.id.search_editText) EditText mSearchText;
@InjectView(R.id.search_button) ImageButton mSearchButton;

protected List<ParseUser> mUsers;

protected final String mUsername = null;

public static final String TAG = SearchActivity.class.getSimpleName();

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

    mGridView = (GridView)findViewById(R.id.friendsGrid);

    mGridView.setChoiceMode(ListView.CHOICE_MODE_MULTIPLE);
    mGridView.setOnItemClickListener(mOnItemClickListener);
}

@Override
protected void onResume(){
    super.onResume();
    ButterKnife.inject(this);

    mCurrentUser = ParseUser.getCurrentUser();
    mFriendsRelation = mCurrentUser.getRelation(ParseConstants.KEY_FRIENDS_RELATION);

    mSearchButton.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            QueryUser();
        }
    });
}

private void QueryUser() {
    EditText gUname;
    gUname = (EditText)findViewById(R.id.search_editText);
    setProgressBarIndeterminateVisibility(true);

    String gUsername = gUname.getText().toString();

    Log.d("singleTune", "Check for "+gUsername);
    ParseQuery<ParseUser> query = ParseUser.getQuery();
    query.whereContains("username", gUsername);
    query.orderByAscending(ParseConstants.KEY_USERNAME);
    query.setLimit(1000);
    query.findInBackground(new FindCallback<ParseUser>() {
        @Override
        public void done(List<ParseUser> users, ParseException e) {
            setProgressBarIndeterminateVisibility(false);
            if(e != null){
                Log.d("singleTune", "Nothing found");
            }else{
                mUsers = users;
                String[] usernames = new String[mUsers.size()];
                int i = 0;
                for(ParseUser user : mUsers){
                    usernames[i] = user.getUsername();
                    i++;
                }
                if(mGridView.getAdapter() == null){
                    UserListAdapter adapter = new UserListAdapter(SearchActivity.this, mUsers);
                    mGridView.setAdapter(adapter);
                }
                else {
                    ((UserListAdapter)mGridView.getAdapter()).refill(mUsers);
                }

                friendFollowCheck();
            }

        }
    });
}



@Override
public boolean onCreateOptionsMenu(Menu menu) {

    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.search_results, 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();
    if (id == R.id.action_settings) {
        return true;
    }
    return super.onOptionsItemSelected(item);
}

private void friendFollowCheck(){
    mFriendsRelation.getQuery().findInBackground(new FindCallback<ParseUser>() {
        @Override
        public void done(List<ParseUser> friends, ParseException e) {
            if(e == null){
                for (int i = 0; i < mUsers.size(); i++){
                    ParseUser user = mUsers.get(i);
                    for (ParseUser friend : friends){
                        if (friend.getObjectId().equals(user.getObjectId())){
                            //TODO: Change button style and text
                            mGridView.setItemChecked(i, true);
                        }
                    }
                }
            }
            else {
                Log.e(TAG, e.getMessage());
            }
        }
    });
}

protected AdapterView.OnItemClickListener mOnItemClickListener = new AdapterView.OnItemClickListener() {
    Boolean followed;
    @Override
    public void onItemClick(AdapterView<?> adapterView, View view, int i, long l) {
    if (mGridView.isItemChecked(i)) {
        mFriendsRelation.add(mUsers.get(i));
        followed = true;
        sendPushNotification(mUsers.get(i).getObjectId(), followed);
        Log.d(TAG, "Checked");
    }
    else {
        // remove the friend
        mFriendsRelation.remove(mUsers.get(i));
        followed = false;
        sendPushNotification(mUsers.get(i).getObjectId(), followed);
    }
    mCurrentUser.saveInBackground(new SaveCallback() {
        @Override
        public void done(ParseException e) {
            if (e != null) {
                Log.e(TAG, e.getMessage());
            }
        }
    });
    }
};

private void sendPushNotification(String userId, Boolean b){
    ParseQuery<ParseInstallation> query = ParseInstallation.getQuery();
    query.whereEqualTo(ParseConstants.KEY_USER_ID, userId);

    ParsePush push = new ParsePush();
    push.setQuery(query);
    if (b){
        push.setMessage("Yay! "+ ParseUser.getCurrentUser().getUsername()+ " is now following you.");
    }
    else{
        push.setMessage("Aww! "+ ParseUser.getCurrentUser().getUsername()+ " has unfollowed you.");
    }
    push.sendInBackground();
}

public class UserListAdapter extends ArrayAdapter<ParseUser> {

private static final String TAG = "Test";
protected Context mContext;
protected List<ParseUser> mParseUsers;
protected ParseRelation<ParseUser> mFriendsRelation;
protected ParseUser mCurrentUser;
protected Boolean followed;
protected Boolean followStatus;

protected List<ParseUser> mUsers;

public UserListAdapter(Context context, List<ParseUser> users){
    super(context, R.layout.user_item, users);
    mContext = context;
    mParseUsers = users;
}

@Override
public View getView(int position, View convertView, ViewGroup parent) {

    ViewHolder holder;

    if (convertView == null){
        LayoutInflater inflater = (LayoutInflater.from(mContext));
        convertView = inflater.inflate(R.layout.user_item, parent, false);
        holder = new ViewHolder();
        holder.avatar = (ImageView) convertView.findViewById(R.id.avatarImageView);
        holder.selectedAvatar = (ImageView) convertView.findViewById(R.id.selectedAvatarImageView);
        holder.usernameLabel = (TextView) convertView.findViewById(R.id.usernameLabel);
        convertView.setTag(holder);
    }
    else {
        holder = (ViewHolder)convertView.getTag();
    }

    ParseUser user = mParseUsers.get(position);

    String email = user.getEmail().toLowerCase();

    if(email.equals("")){
        holder.avatar.setImageResource(R.drawable.default_avatar);
    }
    else{
        String hash = MD5Util.md5Hex(email);
        String gravatarUrl = "http://www.gravatar.com/avatar/"+ hash + "?s=272&d=404";
        Picasso.with(mContext).load(gravatarUrl).placeholder(R.drawable.default_avatar).into(holder.avatar);
    }
    holder.usernameLabel.setText(user.getUsername());

    GridView gridView = (GridView)parent;
    if(gridView.isItemChecked(position)){
        holder.selectedAvatar.setVisibility(View.VISIBLE);
    }
    else {

        holder.selectedAvatar.setVisibility(View.INVISIBLE);
    }

    return convertView;
}



//
private static class ViewHolder{
    ImageView avatar;
    ImageView selectedAvatar;
    TextView usernameLabel;
}

public void refill(List<ParseUser> users){
    mParseUsers.clear();
    mParseUsers.addAll(users);
    notifyDataSetChanged();
}