Compile and run the following program:

---%<---
package comboboxtest;
import java.awt.Component;
import java.awt.EventQueue;
import java.awt.Font;
import javax.swing.DefaultComboBoxModel;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JList;
import javax.swing.ListCellRenderer;
import javax.swing.plaf.FontUIResource;
import javax.swing.tree.DefaultTreeCellRenderer;
public class Main extends JFrame {
    private static final boolean RUN_CORRECTLY = false;
    public Main() {
        initComponents();
        combobox.setModel(new DefaultComboBoxModel(new Object[] {"hello", "goodbye"}));
        combobox.setRenderer(new MyRenderer());
        pack();
    }
    // <editor-fold defaultstate="collapsed" desc=" Generated Code ">
    private void initComponents() {
        irrelevant = new javax.swing.JLabel();
        combobox = new javax.swing.JComboBox();

        getContentPane().setLayout(new java.awt.FlowLayout());

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);
        irrelevant.setText("Here we are:");
        getContentPane().add(irrelevant);

        getContentPane().add(combobox);

        pack();
    }
    // </editor-fold>
    public static void main(String args[]) {
        EventQueue.invokeLater(new Runnable() {
            public void run() {
                new Main().setVisible(true);
            }
        });
    }
    // Variables declaration - do not modify
    private javax.swing.JComboBox combobox;
    private javax.swing.JLabel irrelevant;
    // End of variables declaration
    private static final class MyRenderer extends DefaultTreeCellRenderer implements ListCellRenderer {
        public Component getListCellRendererComponent(JList list, Object value, int index, boolean isSelected, boolean cellHasFocus) {
            setText((String) value);
            setFont(list.getFont());
            return this;
        }
        public void setFont(Font font) {
            if (RUN_CORRECTLY && font instanceof FontUIResource) {
                font = new Font(font.getName(), font.getStyle(), font.getSize());
            }
            super.setFont(font);
        }
    }
}
---%<---

Using Mustang b35, this class runs fine and shows a frame with a combo box with two items.

Using Mustang b36, if you set RUN_CORRECTLY to true, it behaves the same. Otherwise, it throws some exceptions and the window never opens:

---%<---
java version "1.6.0-ea"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.6.0-ea-b36)
Java HotSpot(TM) Client VM (build 1.6.0-ea-b36, mixed mode, sharing)

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
        at sun.font.FontManager.getFont2D(Native Method)
        at sun.font.FontDesignMetrics.initMatrixAndMetrics(FontDesignMetrics.java:330)
        at sun.font.FontDesignMetrics.<init>(FontDesignMetrics.java:323)
        at sun.font.FontDesignMetrics.getMetrics(FontDesignMetrics.java:271)
        at sun.font.FontDesignMetrics.getMetrics(FontDesignMetrics.java:232)
        at java.awt.Component.getFontMetrics(Component.java:2695)
        at javax.swing.JComponent.getFontMetrics(JComponent.java:1652)
        at javax.swing.plaf.basic.BasicLabelUI.getBaseline(BasicLabelUI.java:281)
        at javax.swing.JComponent.getBaseline(JComponent.java:2585)
        at javax.swing.plaf.basic.BasicComboBoxUI.getDisplaySize(BasicComboBoxUI.java:1289)
        at javax.swing.plaf.metal.MetalComboBoxUI.getMinimumSize(MetalComboBoxUI.java:302)
        at javax.swing.plaf.basic.BasicComboBoxUI.getPreferredSize(BasicComboBoxUI.java:863)
        at javax.swing.JComponent.getPreferredSize(JComponent.java:1687)
        at java.awt.FlowLayout.preferredLayoutSize(FlowLayout.java:401)
        at java.awt.Container.preferredSize(Container.java:1559)
        at java.awt.Container.getPreferredSize(Container.java:1544)
        at javax.swing.JComponent.getPreferredSize(JComponent.java:1689)
        at javax.swing.JRootPane$RootLayout.preferredLayoutSize(JRootPane.java:835)
        at java.awt.Container.preferredSize(Container.java:1559)
        at java.awt.Container.getPreferredSize(Container.java:1544)
        at javax.swing.JComponent.getPreferredSize(JComponent.java:1689)
        at java.awt.BorderLayout.preferredLayoutSize(BorderLayout.java:694)
        at java.awt.Container.preferredSize(Container.java:1559)
        at java.awt.Container.getPreferredSize(Container.java:1544)
        at java.awt.Window.pack(Window.java:493)
        at comboboxtest.Main.<init>(Main.java:18)
        at comboboxtest.Main$1.run(Main.java:39)
        at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:209)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:589)
        at java.awt.EventDispatchThread.pumpOneEventForHierarchy(EventDispatchThread.java:247)
        at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:168)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:162)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:154)
        at java.awt.EventDispatchThread.run(EventDispatchThread.java:115)
Exception in thread "AWT-EventQueue-0" Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
        at sun.font.FontManager.getFont2D(Native Method)
        at sun.font.FontDesignMetrics.initMatrixAndMetrics(FontDesignMetrics.java:330)
        at sun.font.FontDesignMetrics.<init>(FontDesignMetrics.java:323)
        at sun.font.FontDesignMetrics.getMetrics(FontDesignMetrics.java:271)
        at sun.font.FontDesignMetrics.getMetrics(FontDesignMetrics.java:232)
        at java.awt.Component.getFontMetrics(Component.java:2695)
        at javax.swing.JComponent.getFontMetrics(JComponent.java:1652)
        at javax.swing.plaf.basic.BasicLabelUI.getBaseline(BasicLabelUI.java:281)
        at javax.swing.JComponent.getBaseline(JComponent.java:2585)
        at javax.swing.plaf.basic.BasicComboBoxUI.getDisplaySize(BasicComboBoxUI.java:1289)
        at javax.swing.plaf.metal.MetalComboBoxUI.getMinimumSize(MetalComboBoxUI.java:302)
        at javax.swing.plaf.basic.BasicComboBoxUI.getPreferredSize(BasicComboBoxUI.java:863)
        at javax.swing.JComponent.getPreferredSize(JComponent.java:1687)
        at java.awt.FlowLayout.layoutContainer(FlowLayout.java:586)
        at java.awt.Container.layout(Container.java:1402)
        at java.awt.Container.doLayout(Container.java:1391)
        at java.awt.Container.validateTree(Container.java:1474)
        at java.awt.Container.validateTree(Container.java:1481)
        at java.awt.Container.validateTree(Container.java:1481)
        at java.awt.Container.validateTree(Container.java:1481)
        at java.awt.Container.validate(Container.java:1449)
        at java.awt.Window.dispatchEventImpl(Window.java:1807)
        at java.awt.Component.dispatchEvent(Component.java:4194)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:591)
        at java.awt.EventDispatchThread.pumpOneEventForHierarchy(EventDispatchThread.java:247)
        at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:168)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:162)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:154)
        at java.awt.EventDispatchThread.run(EventDispatchThread.java:115)
java.lang.NullPointerException
        at sun.font.FontManager.getFont2D(Native Method)
        at sun.font.FontDesignMetrics.initMatrixAndMetrics(FontDesignMetrics.java:330)
        at sun.font.FontDesignMetrics.<init>(FontDesignMetrics.java:323)
        at sun.font.FontDesignMetrics.getMetrics(FontDesignMetrics.java:271)
        at sun.font.FontDesignMetrics.getMetrics(FontDesignMetrics.java:232)
        at java.awt.Component.getFontMetrics(Component.java:2695)
        at javax.swing.JComponent.getFontMetrics(JComponent.java:1652)
        at javax.swing.plaf.basic.BasicLabelUI.getBaseline(BasicLabelUI.java:281)
        at javax.swing.JComponent.getBaseline(JComponent.java:2585)
        at javax.swing.plaf.basic.BasicComboBoxUI.getDisplaySize(BasicComboBoxUI.java:1289)
        at javax.swing.plaf.metal.MetalComboBoxUI.getMinimumSize(MetalComboBoxUI.java:302)
        at javax.swing.plaf.basic.BasicComboBoxUI.getPreferredSize(BasicComboBoxUI.java:863)
        at javax.swing.JComponent.getPreferredSize(JComponent.java:1687)
        at java.awt.FlowLayout.layoutContainer(FlowLayout.java:586)
        at java.awt.Container.layout(Container.java:1402)
        at java.awt.Container.doLayout(Container.java:1391)
        at java.awt.Container.validateTree(Container.java:1474)
        at java.awt.Container.validateTree(Container.java:1481)
        at java.awt.Container.validateTree(Container.java:1481)
        at java.awt.Container.validateTree(Container.java:1481)
        at java.awt.Container.validate(Container.java:1449)
        at java.awt.Window.dispatchEventImpl(Window.java:1807)
        at java.awt.Component.dispatchEvent(Component.java:4194)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:591)
        at java.awt.EventDispatchThread.pumpOneEventForHierarchy(EventDispatchThread.java:247)
        at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:168)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:162)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:154)
        at java.awt.EventDispatchThread.run(EventDispatchThread.java:115)
---%<---
###@###.### 2005-05-18 23:01:55 GMT