Java: Close the parent JFrame to a combobox

//Full Source Code, straight from Java site, compiles/runs great..  Just doesn't close (NOT EXIT) after selection is made.

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.border.*;
import java.util.*;
import java.text.SimpleDateFormat;

/* ComboBoxDemo2.java requires no other files. */
public class ComboBoxDemo2 extends JPanel
                           implements ActionListener {
    static JFrame frame;
    JLabel result;
    String currentPattern;

    public ComboBoxDemo2() {
        setLayout(new BoxLayout(this, BoxLayout.PAGE_AXIS));
        String[] patternExamples = {
                 "dd MMMMM yyyy",
                 "dd.MM.yy",
                 "MM/dd/yy",
                 "yyyy.MM.dd G 'at' hh:mm:ss z",
                 "EEE, MMM d, ''yy",
                 "h:mm a",
                 "H:mm:ss:SSS",
                 "K:mm a,z",
                 "yyyy.MMMMM.dd GGG hh:mm aaa"
                 };

        currentPattern = patternExamples[0];

        //Set up the UI for selecting a pattern.
        JLabel patternLabel1 = new JLabel("Enter the pattern string or");
        JLabel patternLabel2 = new JLabel("select one from the list:");

        JComboBox patternList = new JComboBox(patternExamples);
        patternList.setEditable(true);
        patternList.addActionListener(this);

        //Create the UI for displaying result.
        JLabel resultLabel = new JLabel("Current Date/Time",
                                        JLabel.LEADING); //== LEFT
        result = new JLabel(" ");
        result.setForeground(Color.black);
        result.setBorder(BorderFactory.createCompoundBorder(
             BorderFactory.createLineBorder(Color.black),
             BorderFactory.createEmptyBorder(5,5,5,5)
        ));

        //Lay out everything.
        JPanel patternPanel = new JPanel();
        patternPanel.setLayout(new BoxLayout(patternPanel,
                               BoxLayout.PAGE_AXIS));
        patternPanel.add(patternLabel1);
        patternPanel.add(patternLabel2);
        patternList.setAlignmentX(Component.LEFT_ALIGNMENT);
        patternPanel.add(patternList);

        JPanel resultPanel = new JPanel(new GridLayout(0, 1));
        resultPanel.add(resultLabel);
        resultPanel.add(result);

        patternPanel.setAlignmentX(Component.LEFT_ALIGNMENT);
        resultPanel.setAlignmentX(Component.LEFT_ALIGNMENT);

        add(patternPanel);
        add(Box.createRigidArea(new Dimension(0, 10)));
        add(resultPanel);

        setBorder(BorderFactory.createEmptyBorder(10,10,10,10));

        reformat();
    } //constructor

    public void actionPerformed(ActionEvent e) {
        JComboBox cb = (JComboBox)e.getSource();
        String newSelection = (String)cb.getSelectedItem();
        currentPattern = newSelection;
        reformat();
    }

    /** Formats and displays today's date. */
    public void reformat() {
        Date today = new Date();
        SimpleDateFormat formatter =
           new SimpleDateFormat(currentPattern);
        try {
            String dateString = formatter.format(today);
            result.setForeground(Color.black);
            result.setText(dateString);
        } catch (IllegalArgumentException iae) {
            result.setForeground(Color.red);
            result.setText("Error: " + iae.getMessage());
        }
    }

    /**
     * Create the GUI and show it.  For thread safety,
     * this method should be invoked from the
     * event-dispatching thread.
     */
    private static void createAndShowGUI() {
        //Create and set up the window.
        JFrame frame = new JFrame("ComboBoxDemo2");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        //Create and set up the content pane.
        JComponent newContentPane = new ComboBoxDemo2();
        newContentPane.setOpaque(true); //content panes must be opaque
        frame.setContentPane(newContentPane);

        //Display the window.
        frame.pack();
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        //Schedule a job for the event-dispatching thread:
        //creating and showing this application's GUI.
        javax.swing.SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                createAndShowGUI();
            }
        });
    }
}

I tried that, by putting it at line 80 here:
    public void actionPerformed(ActionEvent e) {
        JComboBox cb = (JComboBox)e.getSource();
        String newSelection = (String)cb.getSelectedItem();
        currentPattern = newSelection;
        reformat();
        frame.dispose(); ////<---------
    }
   


And I got the following errors:
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
        at ComboBoxDemo2.actionPerformed(ComboBoxDemo2.java:80)
        at javax.swing.JComboBox.fireActionEvent(JComboBox.java:1197)
        at javax.swing.JComboBox.setSelectedItem(JComboBox.java:561)
        at javax.swing.JComboBox.setSelectedIndex(JComboBox.java:597)
        at javax.swing.plaf.basic.BasicComboPopup$Handler.mouseReleased(BasicComboPopup.java:808)
        at java.awt.AWTEventMulticaster.mouseReleased(AWTEventMulticaster.java:232)
        at java.awt.Component.processMouseEvent(Component.java:5554)
        at javax.swing.JComponent.processMouseEvent(JComponent.java:3126)
        at apple.laf.AquaComboBoxUI$AquaComboPopup$1.processMouseEvent(AquaComboBoxUI.java:443)
        at java.awt.Component.processEvent(Component.java:5319)
        at java.awt.Container.processEvent(Container.java:2010)