Call of JProgressBar.getPreferredSize() or ProgressBarUI.getPreferredSize() causes NPE on Windows L&F and custom desktop theme. Unfortunately I am not able to reproduce it with any custom desktop theme. But we have several user reports. It causes silent exit of our NetBeans IDE installer. See for example issue http://www.netbeans.org/issues/show_bug.cgi?id=64380 and attachment given there. Here is one exception call stack example:
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException

at java.awt.Dimension.<init>(Dimension.java:91)
at javax.swing.plaf.basic.BasicProgressBarUI.getPreferredSize(BasicProgressBarUI.java:757)
at javax.swing.JComponent.getPreferredSize(JComponent.java:1615)
at com.installshield.wizard.awt.ColumnLayout.layoutContainer(Unknown Source)
at java.awt.Container.layout(Container.java:1401)
at java.awt.Container.doLayout(Container.java:1390)
at java.awt.Container.validateTree(Container.java:1473)
at java.awt.Container.validateTree(Container.java:1480)
at java.awt.Container.validateTree(Container.java:1480)
at java.awt.Container.validateTree(Container.java:1480)
at java.awt.Container.validateTree(Container.java:1480)
at java.awt.Container.validateTree(Container.java:1480)
at java.awt.Container.validateTree(Container.java:1480)
at java.awt.Container.validate(Container.java:1448)
at javax.swing.RepaintManager.validateInvalidComponents(RepaintManager.java:379)
at javax.swing.SystemEventQueueUtilities$ComponentWorkRequest.run(SystemEventQueueUtilities.java:113)
at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:209)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:461)
at java.awt.EventDispatchThread.pumpOneEventForHierarchy(EventDispatchThread.java:242)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:163)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:157)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:149)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:110)

As I checked sources it seems that WindowsProgressBarUI.getPreferredInnerHorizontal() can return null.