error java ketika mau membatasi hanya 1 jinternalframe yang sama yang tampil

Dear All, kode saya berikut mainnya

public class frame3 extends JFrame {

	public JPanel contentPane;

	/**
	 * Launch the application.
	 */
	public static void main(String[] args) {
		EventQueue.invokeLater(new Runnable() {
			public void run() {
				try {
					frame3 frame = new frame3();
					frame.setVisible(true);
				} catch (Exception e) {
					e.printStackTrace();
				}
			}
		});
	}





	/**
	 * Create the frame.
	 */
	public frame3() {
		setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		setBounds(100, 100, 800, 600);
		JDesktopPane desktop = new JDesktopPane();


		JMenuBar menuBar = new JMenuBar();
		setJMenuBar(menuBar);

		JMenu mnDataKaryawan = new JMenu("Data Employee");
		menuBar.add(mnDataKaryawan);

		JMenuItem mntmData = new JMenuItem("Data");
		mntmData.addActionListener(new ActionListener() {
			public void actionPerformed(ActionEvent arg0) {
				data.getInstance().setVisible(true);
				data.getInstance().setBounds(1, 1, 450, 300);
				desktop.add(data.getInstance());

			}
		});




		mnDataKaryawan.add(mntmData);
                contentPane = new JPanel();
		contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));
		setContentPane(contentPane);
		contentPane.setLayout(new CardLayout(0, 0));
		contentPane.add(desktop, "name_19268387084787");
	}
}

lalu

public class data extends JInternalFrame {
	private JTable table;
	static Connection connection = null;
	public void refreshTable(){
		PreparedStatement forquery = null;
		//kondisi PreparedStatement awal kosong(null)
		ResultSet resultr=null;
		//kondisi PreparedStatement awal kosong(null)
		try{
			String queryqry2="select * from karyawan";

			forquery = connection.prepareStatement(queryqry2);
			resultr=forquery.executeQuery();
			table.setModel(DbUtils.resultSetToTableModel(resultr));
			forquery.close();
			resultr.close();
		}
		catch(Exception error){
			error.printStackTrace();
		}
	}
	/**
	 * Launch the application.
	 */
	public static void main(String[] args) {
		EventQueue.invokeLater(new Runnable() {
			public void run() {
				try {

					data frame = new data();
					frame.setVisible(true);
					connection.close();
				} catch (Exception e) {
					e.printStackTrace();
				}
			}
		});
	}

	/**
	 * Create the frame.
	 */
	private static data myInstance;
	public data() {
		connection=xamppkonek.konekdb();
		setResizable(true);
		setMaximizable(true);
		setIconifiable(true);
		setClosable(true);
		setBounds(100, 100, 450, 300);
		getContentPane().setLayout(null);

		JScrollPane scrollPane = new JScrollPane();
		scrollPane.setBounds(10, 11, 414, 165);
		getContentPane().add(scrollPane);

		table = new JTable();
		scrollPane.setViewportView(table);
		refreshTable();

	}


	public static data getInstance() {
		 if (myInstance == null)
		 {
	            myInstance = new data();
	            }
			return myInstance;
	    }

}

dirun bisa cuma ketika saya klik ke dua kalinya di menu data untuk tes tidak berulang ada error java.lang.IllegalArgumentException

catatan = saya menggunakan IDE Eclipse dengan windowbuilder

mohon bantuannya all