帮我看看这个异常是什么原因？

[javalearner]

C:\JBuilder4\jdk1.3\bin\javaw -classpath "D:\JB-myProject\UIDesigner\classes;C:\JBuilder4\lib\jbcl.jar;C:\JBuilder4\jdk1.3\jre\lib\i18n.jar;C:\JBuilder4\jdk1.3\jre\lib\jaws.jar;C:\JBuilder4\jdk1.3\jre\lib\rt.jar;C:\JBuilder4\jdk1.3\jre\lib\sunrsasign.jar;C:\JBuilder4\jdk1.3\lib\dt.jar;C:\JBuilder4\jdk1.3\lib\tools.jar" uidesigner.UIDesigner
java.io.NotSerializableException: com.sun.java.swing.plaf.windows.WindowsFileChooserUI
Exception occurred during event dispatching:
java.lang.NullPointerException
at javax.swing.plaf.basic.BasicComboBoxUI.setPopupVisible(BasicComboBoxUI.java:897)
at javax.swing.JComboBox.setPopupVisible(JComboBox.java:609)
at javax.swing.JComboBox.hidePopup(JComboBox.java:602)
at javax.swing.JComboBox$1.ancestorRemoved(JComboBox.java:148)
at javax.swing.AncestorNotifier.fireAncestorRemoved(AncestorNotifier.java:89)
at javax.swing.AncestorNotifier.componentHidden(AncestorNotifier.java:186)
at java.awt.AWTEventMulticaster.componentHidden(AWTEventMulticaster.java:130)
at java.awt.AWTEventMulticaster.componentHidden(AWTEventMulticaster.java:130)
at java.awt.Component.processComponentEvent(Component.java:3609)
at java.awt.Component.processEvent(Component.java:3558)
at java.awt.Container.processEvent(Container.java:1164)
at java.awt.Window.processEvent(Window.java:777)
at java.awt.Component.dispatchEventImpl(Component.java:2595)
at java.awt.Container.dispatchEventImpl(Container.java:1213)
at java.awt.Window.dispatchEventImpl(Window.java:912)
at java.awt.Component.dispatchEvent(Component.java:2499)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:319)
at java.awt.EventDispatchThread.pumpOneEvent(EventDispatchThread.java:103)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:84)

[weidegong]

java.lang.NullPointerException 字符串、数组、记录集等等没有初始化就被使用。。

[jokerjava]

java.lang.NotSerializableException
没有序列化
如果你要保存对象
或者传递对象
应该实现Serializable接口

[alphazhao]

java.lang.NullPointerException
应该是某个类型的变量没有得到赋值，即为null，而你却对它进行了判断等运算或者其它的处理，因而报错
比如String str;
此时没有赋值，你若int i = str.length()便会报这种错误。当然此时注意，null并不是等同于空值。即此处对String而言，null不同于"";

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在下所说，也许并非正确，请从建议的角度批判地吸收！

[javalearner]

谢谢各位