FATAL EXCEPTION on my Custom ListView adapter

 private void QueryUser() {
        FlatEditText gUname;
        gUname = (FlatEditText)findViewById(R.id.search_editText);

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

        Log.d("singleTune", "Check for "+gUsername);
        ParseQuery<ParseUser> query = ParseUser.getQuery();
        query.whereEqualTo("username", gUsername);
        query.orderByAscending(ParseConstants.KEY_USERNAME);
        query.setLimit(1000);
        query.findInBackground(new FindCallback<ParseUser>() {
            @Override
            public void done(List<ParseUser> users, ParseException e) {
                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++;
                    }
                    UserListAdapter adapter = new UserListAdapter(SearchActivity.this, mUsers);
                    setListAdapter(adapter);
                }

            }
        });
    }

public class UserListAdapter extends ArrayAdapter<ParseUser> {

    protected Context mContext;
    protected List<ParseUser> mParseUsers;

    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) {

        View rowView = convertView;
        ViewHolder holder;

        if (rowView == null){
            rowView = LayoutInflater.from(mContext).inflate(R.layout.user_item, null);
            holder = new ViewHolder();
            holder.avatar = (ImageView)convertView.findViewById(R.id.user_imageView);
            holder.usernameLabel = (TextView)convertView.findViewById(R.id.username_textView);
            holder.followButton = (FlatButton)convertView.findViewById(R.id.item_follow_button);
        }
        else{
            holder = (ViewHolder)convertView.getTag();
        }

        ParseUser user = mParseUsers.get(position);
        Log.d("SingleTune", "Username from query is "+user.getUsername());
        holder.avatar.setImageResource(R.drawable.ic_action_picture);
        holder.usernameLabel.setText(user.getString(ParseConstants.KEY_USERNAME));

        return rowView                                                                                                                                                                                                                                                                                                                                                                                                                                                                      ;
    }

    private static class ViewHolder{
        ImageView avatar;
        TextView usernameLabel;
        FlatButton followButton;
    }
}