combo box not working

failed gui

package main;

import data.Constants;
import data.Variables;

import java.awt.BorderLayout;
import java.awt.EventQueue;

import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.border.EmptyBorder;
import javax.swing.SpringLayout;
import javax.swing.JLabel;
import java.awt.Font;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.SwingConstants;
import javax.swing.JToggleButton;
import javax.swing.JRadioButton;
import javax.swing.JTextField;
import javax.swing.JButton;
import javax.swing.JComboBox;
import javax.swing.DefaultComboBoxModel;

public class UI extends JFrame {

	private JPanel contentPane;
	private JTextField txtPleaseEnterPlayers;

	/**
	 * Launch the application.
	 */
	public static void main(String[] args) {
		EventQueue.invokeLater(new Runnable() {
			public void run() {
				try {
					UI frame = new UI();
					frame.setVisible(true);
				} catch (Exception e) {
					e.printStackTrace();
				}
			}
		});
	}

	/**
	 * Create the frame.
	 */
	public UI() {
		setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		setBounds(100, 100, 450, 300);
		contentPane = new JPanel();
		contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));
		contentPane.setLayout(new BorderLayout(0, 0));
		setContentPane(contentPane);

		JPanel panel = new JPanel();
		contentPane.add(panel, BorderLayout.CENTER);
		panel.setLayout(null);

		JLabel lblLordPrayer = new JLabel("Lord Prayer");
		lblLordPrayer.setBounds(146, 11, 138, 29);
		lblLordPrayer.setHorizontalAlignment(SwingConstants.CENTER);
		lblLordPrayer.setFont(new Font("Myriad Pro Light", Font.PLAIN, 28));


		JRadioButton rdbtnBuryBones = new JRadioButton("Bury Bones");
		rdbtnBuryBones.setBounds(6, 128, 94, 50);
		panel.add(rdbtnBuryBones);

		JRadioButton rdbtnOtherHouse = new JRadioButton("Other house");
		rdbtnOtherHouse.setBounds(6, 181, 85, 42);
		panel.add(rdbtnOtherHouse);

		txtPleaseEnterPlayers = new JTextField();
		txtPleaseEnterPlayers.setText("Please enter players name");
		txtPleaseEnterPlayers.setToolTipText("Other player house name");
		txtPleaseEnterPlayers.setBounds(146, 196, 152, 20);
		panel.add(txtPleaseEnterPlayers);
		txtPleaseEnterPlayers.setColumns(10);

		JComboBox comboBox = new JComboBox();
		comboBox.setModel(new DefaultComboBoxModel(new String[]{"Bones", "Bat Bones", "Big Bones", "Dragon Bones"}));
		comboBox.setBounds(109, 143, 305, 20);
		panel.add(comboBox);


		JButton btnStart = new JButton("Start");
		btnStart.setBounds(6, 229, 418, 23);
		panel.add(btnStart);
		btnStart.addActionListener(new ActionListener() {
			@Override
			public void actionPerformed(ActionEvent e) {
				StartButtonActionPerformed();
			}
		});

	}


	private void StartButtonActionPerformed(){
		switch (comboBox.getSelectedIndex()) {
			case 0:
				Variables.bones = Constants.BONES;
				break;
			case 1:
				Variables.bones = Constants.BAT_BONES;
				break;
			case 2:
				Variables.bones = Constants.BIG_BONES;
				break;
			case 3:
				Variables.bones = Constants.DRAGON_BONES;
				break;
			default:
				System.out.println("Something wrong as happened please contact the script writers.");
				break;
		}
		if (rdbtnBuryBones.isSelected()) {
			Variables.setBurybones(true);
		} else {
			Variables.setBurybones(false);
		}
		Variables.setStartScript(true);
		setVisible(false);
	}

	private final JComboBox comboBox = new JComboBox();
	private final JToggleButton rdbtnBuryBones = new JToggleButton("Power Bury");
}