Don't know how to fix

This is the Problem:
[10:09:53] [Server thread/CRITICAL]: ParseError: "syntax error, unexpected ')', expecting '{'" (EXCEPTION) in "plugins/teleporter/src/fixed/Main" at line 16

And this is my code:

<?php

namespace Server;

use pocketmine\plugin\PluginBase;
use pocketmine\event\Listener;
use pocketmine\command\CommandSender;
use pocketmine\command\Command;
use pocketmine\Player;
use pocketmine\plugin\Plugin;
use jojoe77777\FormAPI\SimpleForm;

class Main extends Listener{

public function openHelpUI(){
$form = new SimpleForm(function(Player $player, int $data = null)) {
$form->addbutton("§3Citybuild");
}
}

public function onCommand(CommandSender $sender, string $commandLabel, array $args) {
switch ($args[0]) {
case "tpuicity":
$this->openHelpUI($sender, "Menu Opened");
}
}

 

PHP:

public function openHelpUI(Player $player,string $unknown){
$form = new SimpleForm(function(Player $player, int $data = null) {});
$form->addbutton("§3Citybuild");
$player->sendForm($form);
}

 

Thanks for Help, but there ist a new Problem:

[20:12:02] [Server thread/CRITICAL]: ParseError: "syntax error, unexpected ' ' (T_STRING), expecting '{'" (EXCEPTION) in "plugins/teleporter/src/fixed/Main" at line 16
[20:12:02] [Server thread/DEBUG]: #0 ():