[JAVA3D] error when using the removechild function

[SUBSCRIBE JAVA3D-INTEREST peter]

deal all,
 i had created a button and when the button is pressed,a object under a BG should be 
remove.
But when compile, the coding is no error. When it is running,it show the error message 
of
 "Exception occurred during event dispatching:
    at TestApplet.actionPerformed(TestApplet.java)
    at java.awt.Button.processActionEvent(Button.java:308)
    at.java.awt.Button.processEvent(Button.java:281);
    at.java.awt.Component.dispatchEventImp(Componet.java)
    .......
.....
...
"
any idea ? thx

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