FULL PRODUCT VERSION :
java version "1.6.0_03"
Java(TM) SE Runtime Environment (build 1.6.0_03-b05)
Java HotSpot(TM) Client VM (build 1.6.0_03-b05, mixed mode, sharing)

ADDITIONAL OS VERSION INFORMATION :
Microsoft Windows XP [Version 5.1.2600]

A DESCRIPTION OF THE PROBLEM :
javax.swing.plaf.basic.BasicComboBoxUI.isPopupVisible(BasicComboBoxUI.java:825) returns NullPointerException when called from a overridden method of getModel().

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
If a class extends JComboBox and Overrides getModel(). Then it calls isPopupVisible() from the overriden getModel() it gets NullPointerException the first time. Because the popup variable in javax.swing.plaf.basic.BasicComboBoxUI is not set yet.
I think that the isPopupVisible() should have an "if (popup == null) return false;". Because the menu isn't visible yet, if it is not even setup.

ACTUAL -
NullPointerException

ERROR MESSAGES/STACK TRACES THAT OCCUR :
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
        at javax.swing.plaf.basic.BasicComboBoxUI.isPopupVisible(BasicComboBoxUI.java:825)
        at javax.swing.JComboBox.isPopupVisible(JComboBox.java:807)
        at combo.getModel(combo.java:29)
        at javax.swing.plaf.basic.BasicComboPopup.createList(BasicComboPopup.java:467)
        at javax.swing.plaf.basic.BasicComboPopup.<init>(BasicComboPopup.java:313)
        at javax.swing.plaf.basic.BasicComboBoxUI.createPopup(BasicComboBoxUI.java:393)
        at javax.swing.plaf.metal.MetalComboBoxUI.createPopup(MetalComboBoxUI.java:148)
        at javax.swing.plaf.basic.BasicComboBoxUI.installUI(BasicComboBoxUI.java:217)
        at javax.swing.JComponent.setUI(JComponent.java:673)
        at javax.swing.JComboBox.setUI(JComboBox.java:238)
        at javax.swing.JComboBox.updateUI(JComboBox.java:247)
        at javax.swing.JComboBox.init(JComboBox.java:212)
        at javax.swing.JComboBox.<init>(JComboBox.java:206)
        at combo.<init>(combo.java:24)
        at NewJFrame.initComponents(NewJFrame.java:29)
        at NewJFrame.<init>(NewJFrame.java:17)
        at NewJFrame$2.run(NewJFrame.java:77)
        at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:209)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:597)
        at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:273)
        at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:183)
        at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:173)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:168)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:160)
        at java.awt.EventDispatchThread.run(EventDispatchThread.java:121)


REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
import javax.swing.ComboBoxModel;
import javax.swing.JComboBox;

// use this JComboBox in a frame

public class combo extends JComboBox
{
    public combo()
    {
super();
    }
    @Override
    public ComboBoxModel getModel()
    {
isPopupVisible();
return super.getModel();
    }
}
---------- END SOURCE ----------