FULL PRODUCT VERSION :
java version "1.6.0_10-beta"
Java(TM) SE Runtime Environment (build 1.6.0_10-beta-b22)
Java HotSpot(TM) Client VM (build 11.0-b11, mixed mode, sharing)

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

A DESCRIPTION OF THE PROBLEM :
Custom SliderUI throws a NullPointerException when displayed with Nimbus look and feel.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
A slider as with java TestCustomSlider
ACTUAL -
Exception in thread "main" java.lang.NullPointerException
        at javax.swing.plaf.basic.BasicSliderUI.calculateContentRect(Unknown Source)

ERROR MESSAGES/STACK TRACES THAT OCCUR :
C:\Java\projects\playground>java -Dswing.defaultlaf=com.sun.java.swing.plaf.nimb
us.NimbusLookAndFeel TestCustomSlider
Exception in thread "main" java.lang.NullPointerException
        at javax.swing.plaf.basic.BasicSliderUI.calculateContentRect(Unknown Source)
        at javax.swing.plaf.basic.BasicSliderUI.calculateGeometry(Unknown Source)
        at javax.swing.plaf.basic.BasicSliderUI.installUI(Unknown Source)
        at javax.swing.JComponent.setUI(Unknown Source)
        at javax.swing.JSlider.setUI(Unknown Source)
        at TestCustomSlider$1.updateUI(TestCustomSlider.java:14)
        at javax.swing.JSlider.<init>(Unknown Source)
        at javax.swing.JSlider.<init>(Unknown Source)
        at TestCustomSlider$1.<init>(TestCustomSlider.java:12)
        at TestCustomSlider.<init>(TestCustomSlider.java:12)
        at TestCustomSlider.main(TestCustomSlider.java:24)

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
import java.awt.*;
import javax.swing.*;
import javax.swing.plaf.ComponentUI;
import javax.swing.plaf.basic.BasicSliderUI;

// javac TestCustomSlider.java
// java -Dswing.defaultlaf=com.sun.java.swing.plaf.nimbus.NimbusLookAndFeel TestCustomSlider
public class TestCustomSlider {

    public TestCustomSlider() {
        JFrame mainFrame = new JFrame();
	JSlider jsTortle = new JSlider() {
            public void updateUI() {
                setUI((LVSliderUI)LVSliderUI.createUI(this));
            }
        };
	mainFrame.getContentPane().add(jsTortle);
	mainFrame.pack();
	mainFrame.setVisible(true);
	mainFrame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
    }
    
    public static void main(String[] args) {
	new TestCustomSlider();
    }
    
    public static class LVSliderUI extends BasicSliderUI {
    
	    public static ComponentUI createUI(JComponent c)    {
		return new LVSliderUI();
	    }
	
	    public LVSliderUI() {
		super(null);
	    }
	    
	    public void paintThumb(Graphics g)  {
		int x = slider.getValue()*slider.getSize().width/100;
		g.setColor(Color.RED);
		g.drawRect(x, 0, 10, 10);
	    }
	    public void paintTrack(Graphics g)  {
		g.setColor(Color.BLUE);
		g.drawLine(0,5,slider.getSize().width, 5);
	    }
    }
}
---------- END SOURCE ----------