applet启动时的随机错误

我的applet启动时可能会出现一个错误
信息如下
java.lang.NullPointerException
at javax.swing.plaf.basic.BasicSliderUI.getHeightOfTallestLabel(BasicSliderUI.java:624)
at javax.swing.plaf.basic.BasicSliderUI.calculateLabelRect(BasicSliderUI.java:521)
at javax.swing.plaf.basic.BasicSliderUI.calculateGeometry(BasicSliderUI.java:355)
at javax.swing.plaf.basic.BasicSliderUI$ComponentHandler.componentResized(BasicSliderUI.java:1555)
at java.awt.Component.processComponentEvent(Component.java:4958)
at java.awt.Component.processEvent(Component.java:4912)
at java.awt.Container.processEvent(Container.java:1569)
at java.awt.Component.dispatchEventImpl(Component.java:3615)
at java.awt.Container.dispatchEventImpl(Container.java:1627)
at java.awt.Component.dispatchEvent(Component.java:3477)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:456)
at java.awt.EventDispatchThread.pumpOneEventForHierarchy(EventDispatchThread.java:201)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:151)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:145)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:137)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:100)这个错误不是每次必定出现的请问错误的原因是什么呢