JPanel adden op een JFrame in een andere class

package test;

import java.awt.event.InputEvent;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;

import javax.swing.JMenu;
import javax.swing.JMenuBar;
import javax.swing.JMenuItem;
import javax.swing.KeyStroke;

public class MenuGUI {

    private JMenuBar menuBar;

        /** Constructs the menu bar. */
    public MenuGUI() {

        menuBar = new JMenuBar();

        JMenu consultatie = new JMenu("Consultatie");
        menuBar.add(consultatie);
        JMenuItem nieuweClientItem= new JMenuItem("Nieuwe Cliënt");
        nieuweClientItem.setAccelerator(KeyStroke.getKeyStroke('C', InputEvent.CTRL_DOWN_MASK));
        consultatie.add(nieuweClientItem);

        JMenu boekhouding = new JMenu("Boekhouding");
        menuBar.add(boekhouding);

        nieuweClientItem.addActionListener (
                new ActionListener() {
                    public void actionPerformed(ActionEvent e) {

                        NieuweClientGUI nieuweClientGUI = new NieuweClientGUI();

                        [COLOR="Red"]frame.[/COLOR]add(nieuweClientGUI.getPanel());

                    }
                }
        );

    }

        /** Returns the menu bar corresponding to this menu. */
    public JMenuBar getMenuBar() {return menuBar;}

}

package test;

import javax.swing.JFrame;

public class Main{

    private static void createAndShowGUI(){

        JFrame frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            // construct a Menu object...
        MenuGUI menu = new MenuGUI();
            // ...and obtain its menu bar
        frame.setJMenuBar(menu.getMenuBar());

        frame.pack();
        frame.setSize(500,500);
        frame.setVisible(true);

    }

    public static void main(String[] args) {

        javax.swing.SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                createAndShowGUI();
            }
        });
    }
}

public MenuGUI(JFrame frame) {
this.frame = frame;

//rest van code
}

//...
MenuGUI menu = new MenuGui(frame);
frame.setJMenuBar(menu.getMenuBar());
//...