FULL PRODUCT VERSION :
java version "1.6.0-beta2"
Java(TM) SE Runtime Environment (build 1.6.0-beta2-b84)
Java HotSpot(TM) 64-Bit Server VM (build 1.6.0-beta2-b84, mixed mode)

ADDITIONAL OS VERSION INFORMATION :
Red Hat Linux Fedora Core 4
custom 2.6.16 kernel x86_64 GNU/Linux

EXTRA RELEVANT SYSTEM CONFIGURATION :
Dual head X-Windows system (non-Xinerama)

A DESCRIPTION OF THE PROBLEM :
Hiding a modal dialog (e.g. JDialog.dispose()) that is the child of a non-modal dialog, which in turn is the child of a JFrame causes a NullPointerException if the windows are on a secondary screen of a multi-head x-windows configuration (non-Xinerama).

Tested with JOptionPane.showMessageDialog and JFileChooser. Based on trace info, this probably happens on any Dialog.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Run the attached test program. Press the "Open Dialog" button. Press OK on the message dialog.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Dialog closes.
ACTUAL -
Primary screen- dialog closes.
Secondary screen: NullPointerException thrown, dialog stays up.

ERROR MESSAGES/STACK TRACES THAT OCCUR :
Exception occurred during event dispatching:
java.lang.NullPointerException
        at sun.awt.X11.XWindowPeer.restoreTransientFor(XWindowPeer.java:1199)
        at sun.awt.X11.XWindowPeer.removeFromTransientFors(XWindowPeer.java:1265)
        at sun.awt.X11.XWindowPeer.setModalBlocked(XWindowPeer.java:1020)
        at java.awt.Window.setModalBlocked(Window.java:1159)
        at java.awt.Dialog.unblockWindow(Dialog.java:1538)
        at java.awt.Dialog.modalHide(Dialog.java:1442)
        at java.awt.Dialog.hideAndDisposePreHandler(Dialog.java:1171)
        at java.awt.Dialog.hide(Dialog.java:1213)
        at java.awt.Component.show(Component.java:1418)
        at java.awt.Component.setVisible(Component.java:1369)
        at java.awt.Window.setVisible(Window.java:694)
        at java.awt.Dialog.setVisible(Dialog.java:979)
        at javax.swing.JOptionPane$3.propertyChange(JOptionPane.java:997)
        at java.beans.PropertyChangeSupport.firePropertyChange(PropertyChangeSupport.java:339)
        at java.beans.PropertyChangeSupport.firePropertyChange(PropertyChangeSupport.java:276)
        at java.awt.Component.firePropertyChange(Component.java:7802)
        at javax.swing.JOptionPane.setValue(JOptionPane.java:1940)
        at javax.swing.plaf.basic.BasicOptionPaneUI$ButtonActionListener.actionPerformed(BasicOptionPaneUI.java:1184)
        at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:1995)
        at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2318)
        at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:377)
        at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:232)
        at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:236)
        at java.awt.Component.processMouseEvent(Component.java:5999)
        at javax.swing.JComponent.processMouseEvent(JComponent.java:3203)
        at java.awt.Component.processEvent(Component.java:5764)
        at java.awt.Container.processEvent(Container.java:1984)
        at java.awt.Component.dispatchEventImpl(Component.java:4407)
        at java.awt.Container.dispatchEventImpl(Container.java:2042)
        at java.awt.Component.dispatchEvent(Component.java:4237)
        at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4248)
        at java.awt.LightweightDispatcher.processMouseEvent(Container.java:3912)
        at java.awt.LightweightDispatcher.dispatchEvent(Container.java:3842)
        at java.awt.Container.dispatchEventImpl(Container.java:2028)
        at java.awt.Window.dispatchEventImpl(Window.java:2300)
        at java.awt.Component.dispatchEvent(Component.java:4237)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:600)
        at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:273)
        at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:183)
        at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:177)
        at java.awt.Dialog$1.run(Dialog.java:1039)
        at java.awt.Dialog$2.run(Dialog.java:1085)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.awt.Dialog.show(Dialog.java:1083)
        at javax.swing.JOptionPane.showOptionDialog(JOptionPane.java:848)
        at javax.swing.JOptionPane.showMessageDialog(JOptionPane.java:645)
        at javax.swing.JOptionPane.showMessageDialog(JOptionPane.java:616)
        at TestCase$ButtonActionListener.actionPerformed(TestCase.java:35)
        at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:1995)
        at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2318)
        at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:377)
        at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:232)
        at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:236)
        at java.awt.Component.processMouseEvent(Component.java:5999)
        at javax.swing.JComponent.processMouseEvent(JComponent.java:3203)
        at java.awt.Component.processEvent(Component.java:5764)
        at java.awt.Container.processEvent(Container.java:1984)
        at java.awt.Component.dispatchEventImpl(Component.java:4407)
        at java.awt.Container.dispatchEventImpl(Container.java:2042)
        at java.awt.Component.dispatchEvent(Component.java:4237)
        at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4248)
        at java.awt.LightweightDispatcher.processMouseEvent(Container.java:3912)
        at java.awt.LightweightDispatcher.dispatchEvent(Container.java:3842)
        at java.awt.Container.dispatchEventImpl(Container.java:2028)
        at java.awt.Window.dispatchEventImpl(Window.java:2300)
        at java.awt.Component.dispatchEvent(Component.java:4237)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:600)
        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 java.awt.*;
import javax.swing.*;
import java.awt.event.*;

public class TestCase {
        public TestCase() {
                GraphicsEnvironment ge = GraphicsEnvironment.getLocalGraphicsEnvironment();
                GraphicsDevice[] gs = ge.getScreenDevices();

                if (gs.length<2) {
                        System.out.println("Not multi-head environment, test not valid!");
                        System.exit(1);
                }

                for (int i=0; i<gs.length; i++) {
                        JFrame frame=new JFrame("Main App",gs[i].getDefaultConfiguration());
                        frame.show();
                        JDialog dialog=new JDialog(frame,"Dialog "+i,false,gs[i].getDefaultConfiguration());
                        JButton button=new JButton("Open Dialog");
                        button.addActionListener(new ButtonActionListener(dialog));
                        dialog.getContentPane().add(button);
                        dialog.pack();
                        dialog.show();
                }
        }
        private static class ButtonActionListener implements ActionListener {
                JDialog dialog;
                public ButtonActionListener(JDialog dialog) {
                        super();
                        this.dialog=dialog;
                }
                public void actionPerformed(ActionEvent e) {
                        JOptionPane.showMessageDialog(dialog,"Null Pointer Exception Ahead!","Press OK for NPE",JOptionPane.INFORMATION_MESSAGE);
                }
        }
        public static void main(String args[]) {
                new TestCase();
        }
}

---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Use Java version <1.6