Open another Java form using Menubar

Question

Here is my code. I want to open another Java form using menu bar but when I click on menu item, it shows me an error:

Exception in thread "AWT-EventQueue-0" java.lang.IllegalArgumentException: 
    adding a window to a container

What should I do now?

package Driver;

import java.awt.BorderLayout;
import java.awt.EventQueue;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;

import javax.swing.JDesktopPane;
import javax.swing.JFrame;
import javax.swing.JMenu;
import javax.swing.JMenuBar;
import javax.swing.JMenuItem;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.border.EmptyBorder;

public class frmTestMenu extends JFrame {

    private static final long serialVersionUID = 1L;
    private JPanel contentPane;

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

    /**
     * Create the frame.
     */
    public frmTestMenu() {
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setBounds(100, 100,653, 425);
        contentPane = new JPanel();
        contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));
        setContentPane(contentPane);
        contentPane.setLayout(null);

        JMenuBar menuBar = new JMenuBar();
        menuBar.setBounds(0, 0, 434, 31);
        contentPane.add(menuBar);

        JMenu mnFile = new JMenu("FIle");
        menuBar.add(mnFile);

        JMenuItem mntmExit = new JMenuItem("Exit");
        mntmExit.addMouseListener(new MouseAdapter() {
            @Override
            public void mousePressed(MouseEvent e) {
                frmTestMenu.this.dispose();
            }
        });

        mnFile.add(mntmExit);

        JMenu mnItem = new JMenu("Item1");
        menuBar.add(mnItem);

        JMenuItem mntmMenuItem = new JMenuItem("open Test Internal Form");

        mnItem.add(mntmMenuItem);

        JMenuItem mntmMenuItem_3 = new JMenuItem("open Test Internal Form 2");

        mnItem.add(mntmMenuItem_3);

        JMenu mnItem_1 = new JMenu("item 2");
        menuBar.add(mnItem_1);

        JMenuItem mntmMenuItem_1 = new JMenuItem("Menu 2 item 1");
        mnItem_1.add(mntmMenuItem_1);

        JMenu mnItem_2 = new JMenu("item 3");
        menuBar.add(mnItem_2);

        JMenuItem mntmMenuItem_2 = new JMenuItem("Menu 3 item 1");
        mnItem_2.add(mntmMenuItem_2);


        final JDesktopPane desktopPane = new JDesktopPane();
        desktopPane.setBounds(0, 0, 633, 366);
        mntmMenuItem.addMouseListener(new MouseAdapter() {
            @Override 
            public void mousePressed(MouseEvent arg0) {
                frm3_0 test1 = new frm3_0();
                test1.setBounds(10, 10,  426, 229);
                desktopPane.add(test1);
                test1.setVisible(true);
            }
        });
        mntmMenuItem_3.addMouseListener(new MouseAdapter() {
            @Override
            public void mousePressed(MouseEvent e) {
                frmcity_A test2 = new frmcity_A();
                test2.setBounds(10, 10,  426, 229);
                desktopPane.add(test2);
                test2.setVisible(true);
            }
        });
        JScrollPane scrollPane = new JScrollPane(desktopPane);

        JDesktopPane desktopPane_1 = new JDesktopPane();
        desktopPane_1.setBounds(145, 88, 1, 1);
        desktopPane.add(desktopPane_1);
        scrollPane.setBounds(0, 30, 633, 336);
        contentPane.add(scrollPane);

    }
}