WindowsTreeUI.ensureRowsAreVisible does check for null inside the loop (line 92) but it doesn't check for null after the loop (line 96).

java.lang.NullPointerException
        at com.sun.java.swing.plaf.windows.WindowsTreeUI.ensureRowsAreVisible(WindowsTreeUI.java:96)
        at javax.swing.plaf.basic.BasicTreeUI.toggleExpandState(BasicTreeUI.java:2270)
        at javax.swing.plaf.basic.BasicTreeUI.handleExpandControlClick(BasicTreeUI.java:2253)
        at javax.swing.plaf.basic.BasicTreeUI.checkForClickInExpandControl(BasicTreeUI.java:2211)
        at javax.swing.plaf.basic.BasicTreeUI$Handler.handleSelection(BasicTreeUI.java:3580)
        at javax.swing.plaf.basic.BasicTreeUI$Handler.mousePressedDND(BasicTreeUI.java:3566)
        at javax.swing.plaf.basic.BasicTreeUI$Handler.mousePressed(BasicTreeUI.java:3525)
        at java.awt.AWTEventMulticaster.mousePressed(AWTEventMulticaster.java:280)
        at java.awt.Component.processMouseEvent(Component.java:6502)
        at javax.swing.JComponent.processMouseEvent(JComponent.java:3321)
        at java.awt.Component.processEvent(Component.java:6270)
        at java.awt.Container.processEvent(Container.java:2229)
        at java.awt.Component.dispatchEventImpl(Component.java:4861)
        at java.awt.Container.dispatchEventImpl(Container.java:2287)
        at java.awt.Component.dispatchEvent(Component.java:4687)
        at java.awt.LightweightDispatcher.retargetMouseEvent(LightweightDispatcher.java:4832)
        at java.awt.LightweightDispatcher.processMouseEvent(LightweightDispatcher.java:4489)
        at java.awt.LightweightDispatcher.dispatchEvent(LightweightDispatcher.java:4422)
        at java.awt.Container.dispatchEventImpl(Container.java:2273)
        at java.awt.Window.dispatchEventImpl(Window.java:2719)
        at java.awt.Component.dispatchEvent(Component.java:4687)
        at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:723)
        at java.awt.EventQueue.access$200(EventQueue.java:103)
        at java.awt.EventQueue$3.run(EventQueue.java:682)
        at java.awt.EventQueue$3.run(EventQueue.java:680)
        at java.security.AccessController.doPrivileged(AccessController.java:0)
        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:696)
        at java.awt.EventQueue$4.run(EventQueue.java:694)
        at java.security.AccessController.doPrivileged(AccessController.java:0)
        at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:693)
        at org.netbeans.core.TimableEventQueue.dispatchEvent(TimableEventQueue.java:158)
        at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:244)
        at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:163)
        at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:151)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:147)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:139)
        at java.awt.EventDispatchThread.run(EventDispatchThread.java:97)

See also this NetBeans bug: http://netbeans.org/bugzilla/show_bug.cgi?id=213568