FULL PRODUCT VERSION :
java version "1.4.2_03"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.4.2_03-b02)
Java HotSpot(TM) Client VM (build 1.4.2_03-b02, mixed mode)

ADDITIONAL OS VERSION INFORMATION :
Red Hat 7.3 (Linux 2.4.18-17.7.x i686 unknown)


A DESCRIPTION OF THE PROBLEM :
The javadocs state the JTabbedPane.setTitleAt(index, string) can take null as the string argument. However, when you try to start an application the following stack trace is produced:

Exception in thread "main" java.lang.NullPointerException
        at sun.awt.font.FontDesignMetrics.stringWidth(FontDesignMetrics.java:281)
        at javax.swing.SwingUtilities.computeStringWidth(SwingUtilities.java:713)
        at javax.swing.plaf.basic.BasicTabbedPaneUI.calculateTabWidth(BasicTabbedPaneUI.java:1455)
        at javax.swing.plaf.basic.BasicTabbedPaneUI$TabbedPaneLayout.calculateTabRects(BasicTabbedPaneUI.java:2251)
        at javax.swing.plaf.basic.BasicTabbedPaneUI$TabbedPaneLayout.calculateLayoutInfo(BasicTabbedPaneUI.java:2179)
        at javax.swing.plaf.basic.BasicTabbedPaneUI$TabbedPaneLayout.layoutContainer(BasicTabbedPaneUI.java:2094)
        at java.awt.Container.layout(Container.java:1020)
        at java.awt.Container.doLayout(Container.java:1010)
        at java.awt.Container.validateTree(Container.java:1092)
        at java.awt.Container.validateTree(Container.java:1099)
        at java.awt.Container.validateTree(Container.java:1099)
        at java.awt.Container.validateTree(Container.java:1099)
        at java.awt.Container.validateTree(Container.java:1099)
        at java.awt.Container.validate(Container.java:1067)
        at java.awt.Window.show(Window.java:461)
        at TabbedPaneBug.main(TabbedPaneBug.java:14)


STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Create a tabbed pane and try to set a title to null

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The application to run as normal with no title for the tab. Even when you also specify an icon to be displayed on the tab the same error occurs.
ACTUAL -
The component does not appear and a NullPointerException is thrown.

ERROR MESSAGES/STACK TRACES THAT OCCUR :
Exception in thread "main" java.lang.NullPointerException
        at sun.awt.font.FontDesignMetrics.stringWidth(FontDesignMetrics.java:281)
        at javax.swing.SwingUtilities.computeStringWidth(SwingUtilities.java:713)
        at javax.swing.plaf.basic.BasicTabbedPaneUI.calculateTabWidth(BasicTabbedPaneUI.java:1455)
        at javax.swing.plaf.basic.BasicTabbedPaneUI$TabbedPaneLayout.calculateTabRects(BasicTabbedPaneUI.java:2251)
        at javax.swing.plaf.basic.BasicTabbedPaneUI$TabbedPaneLayout.calculateLayoutInfo(BasicTabbedPaneUI.java:2179)
        at javax.swing.plaf.basic.BasicTabbedPaneUI$TabbedPaneLayout.layoutContainer(BasicTabbedPaneUI.java:2094)
        at java.awt.Container.layout(Container.java:1020)
        at java.awt.Container.doLayout(Container.java:1010)
        at java.awt.Container.validateTree(Container.java:1092)
        at java.awt.Container.validateTree(Container.java:1099)
        at java.awt.Container.validateTree(Container.java:1099)
        at java.awt.Container.validateTree(Container.java:1099)
        at java.awt.Container.validateTree(Container.java:1099)
        at java.awt.Container.validate(Container.java:1067)
        at java.awt.Window.show(Window.java:461)
        at TabbedPaneBug.main(TabbedPaneBug.java:14)



REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
import javax.swing.*;

public class TabbedPaneBug
{

  public static void main(String[] args)
  {
     JTabbedPane tabbedPane = new JTabbedPane();
     tabbedPane.add("Tab 1", new JLabel("Test label"));
     tabbedPane.add("Tab 2", new JLabel("Test label 2"));
     tabbedPane.setTitleAt(1, null);
     JFrame frame = new JFrame();
     frame.getContentPane().add(tabbedPane);
     frame.show();
  }

}
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Instead of setting the title to null set it to be an empty string.