Problemas com JInternalFrame

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class SimpleInternalFrame extends Frame implements ActionListener{
	private JButton openButton, winButton, javaButton, motifButton;
	private JLayeredPane desktop;
	JInternalFrame internalFrame;
	
	public SimpleInternalFrame() {
		super("Internal Frame Demo");
		setSize(500, 400);
		openButton = new JButton("Open");
		winButton = new JButton("Windows");
		javaButton = new JButton("Metal");
		motifButton = new JButton("Motif");
		Panel p = new Panel();
		p.add(openButton);
		p.add(winButton);
		p.add(javaButton);
		p.add(motifButton);
		add(p, BorderLayout.SOUTH);
		addWindowListener(new BasicWindowMonitor());
		openButton.addActionListener(this);
		LnFListener lnf = new LnFListener(this);
		winButton.addActionListener(lnf);
		javaButton.addActionListener(lnf);
		motifButton.addActionListener(lnf);
		
		desktop = new JDesktopPane();
		desktop.setOpaque(true);
		add(desktop, BorderLayout.CENTER);
	}
	
	public void actionPerformed(ActionEvent e) {
		if((internalFrame == null) || (internalFrame.isClosed())) {
			internalFrame = new JInternalFrame("Internal Frame", true, true, true, true);
			internalFrame.setBounds(50, 50, 200, 100);
			desktop.add(internalFrame, new Integer(1));
		}
	}
	
	public static void main(String[] args) {
		SimpleInternalFrame sif = new SimpleInternalFrame();
		sif.setVisible(true);
	}
}

import java.awt.event.*;
import java.awt.Window;

public class BasicWindowMonitor extends WindowAdapter{
	public void windowClosing(WindowEvent e) {
		Window w = e.getWindow();
		w.setVisible(false);
		w.dispose();
		System.exit(0);
	}
}

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class LnFListener implements ActionListener{
	private Frame frame;
	
	public LnFListener(Frame f) {
		frame = f;
	}
	
	public void actionPerformed(ActionEvent e) {
		String lnfName = null;
		if(e.getActionCommand().equals("Metal")) lnfName = "javax.swing.plaf.metal.MetalLookAndFeel";
		else if(e.getActionCommand().equals("Motif")) lnfName = "com.sun.java.swing.plaf.motif.MotifLookAndFeel";
		else lnfName = "com.sun.java.swing.plaf.windows.WindowsLookAndFeel";
		
		try {
			UIManager.setLookAndFeel(lnfName);
			SwingUtilities.updateComponentTreeUI(frame);
		}
		catch(UnsupportedLookAndFeelException ex1) {
			System.err.println("Unsipported LookAndFeel: " + lnfName);
		}
		catch(ClassNotFoundException ex2) {
			System.err.println("LookAndFeel class not found: " + lnfName);
		}
		catch(InstantiationException ex3) {
			System.err.println("Could not loaadLookAndFeel: " + lnfName);
		}
		catch(IllegalAccessException ex4) {
			System.err.println("Cannot use loaadLookAndFeel: " + lnfName);
		}
	}
}