Modal dialogs which were launched by JButton cause NULLPointerException error when it is closed by setVisible(false)

As a result of setVisible(false), dialogs disappear but cause an Exception error as follows:

java.lang.NullPointerException at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:1416)
        at java.awt.LightweightDispatcher.processMouseEvent(Container.java:1329)
        at java.awt.LightweightDispatcher.dispatchEvent(Container.java:1264)
        at java.awt.Container.dispatchEventImpl(Container.java:824)
        at java.awt.Window.dispatchEventImpl(Window.java:401)
        at java.awt.Component.dispatchEvent(Component.java:1393)
        at java.awt.EventDispatchThread.run(EventDispatchThread.java:63)
 
Here is test-code that causes exception.

// test-code
import java.awt.*;
import java.awt.event.*;
import com.sun.java.swing.*;

public class DialogWindow extends JFrame implements ActionListener{
    private AboutFrame dialog;

    public DialogWindow() {
        super("test");

//**** if Button is used then no exception under solaris ****//
        JButton button = new JButton("Click to bring up dialog");
        button.addActionListener(this);
        add(button);
        addWindowListener(wl);
    }

    WindowListener wl = new WindowAdapter(){
        public void windowClosing(WindowEvent event){
            System.exit(0);
        }
    };


    public void actionPerformed(ActionEvent event) {
        if(dialog == null) dialog = new AboutFrame(this);
        dialog.setVisible(true);
    }

    public static void main(String args[]) {
        DialogWindow window = new DialogWindow();

        window.pack();
        window.setVisible(true);
    }
}


class AboutFrame extends Dialog implements ActionListener
{
    JPanel upperPanel;
    JPanel lowerPanel;

    public AboutFrame(JFrame frame){
        super(frame, "Cool AWT Test Team!!!", true);
        setSize(300,200);

        upperPanel = new JPanel(true);
        lowerPanel = new JPanel(true);
        lowerPanel.setLayout(new FlowLayout(FlowLayout.CENTER));

        JButton ok = new JButton("OK");
        ok.setActionCommand("OK");
        ok.addActionListener(this);

        lowerPanel.add(ok);

        add("Center", upperPanel);
        add("South", lowerPanel);
    }

    public void actionPerformed(ActionEvent e){
        if((e.getActionCommand()).equals("OK")){
            setVisible(false); // cause exception.
        }
   }

}
 
// The end of the test-code.

bae-chul.kim@Eng 1997-06-25