SYNOPSIS
--------
NullPointerException when sorting JTable with empty cell

OPERATING SYSTEM
----------------
Reported on RHEL5.1
Reproducible on Windows.
Likely platform independent.

FULL JDK VERSION
----------------
JDK 7 (tested with b144)
Not reproducible with Java 6

REPRODUCTION INSTRUCTIONS
-------------------------
1. Launch SwingSet2 with jar as follows:
    java -jar %JAVA_HOME%/demo/jfc/SwingSet2/SwingSet2.jar

2. Click JTableDemo Tab and double click on a cell in the "Favorite
 Number" column and delete the value.

3. Click on the column headers to sort the table.

Expected result:
No Exceptions should be thrown.

Observed result:
The following NullPointerException stack trace is printed on the console:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
      at javax.swing.JTable.columnMoved(JTable.java:4578)
      at javax.swing.table.DefaultTableColumnModel.fireColumnMoved(DefaultTableColumnModel.java:580)
      at javax.swing.table.DefaultTableColumnModel.moveColumn(DefaultTableColumnModel.java:190)
      at javax.swing.plaf.basic.BasicTableHeaderUI$MouseInputHandler.setDraggedDistance(BasicTableHeaderUI.java:293)
      at javax.swing.plaf.basic.BasicTableHeaderUI$MouseInputHandler.mouseReleased(BasicTableHeaderUI.java:263)
      at java.awt.AWTEventMulticaster.mouseReleased(AWTEventMulticaster.java:290)
      at java.awt.Component.processMouseEvent(Component.java:6504)
      at javax.swing.JComponent.processMouseEvent(JComponent.java:3321)
      at java.awt.Component.processEvent(Component.java:6269)
      at java.awt.Container.processEvent(Container.java:2229)
      at java.awt.Component.dispatchEventImpl(Component.java:4860)
      at java.awt.Container.dispatchEventImpl(Container.java:2287)
      at java.awt.Component.dispatchEvent(Component.java:4686)
      at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4832)
      at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4492)
      at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4422)
      at java.awt.Container.dispatchEventImpl(Container.java:2273)
      at java.awt.Window.dispatchEventImpl(Window.java:2713)
      at java.awt.Component.dispatchEvent(Component.java:4686)
      at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:707)
      at java.awt.EventQueue.access$000(EventQueue.java:101)
      at java.awt.EventQueue$3.run(EventQueue.java:666)
      at java.awt.EventQueue$3.run(EventQueue.java:664)
      at java.security.AccessController.doPrivileged(Native Method)
      at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
      at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:87)
      at java.awt.EventQueue$4.run(EventQueue.java:680)
      at java.awt.EventQueue$4.run(EventQueue.java:678)
      at java.security.AccessController.doPrivileged(Native Method)
      at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
      at java.awt.EventQueue.dispatchEvent(EventQueue.java:677)
      at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:211)
      at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:128)
      at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:117)
      at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:113)
      at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:105)
      at java.awt.EventDispatchThread.run(EventDispatchThread.java:90)