Java错误:IllegalArgumentException:向容器添加窗口。

I keep receiving the error:

我不断地收到错误:

Exception in thread "main" java.lang.IllegalArgumentException: adding a window to a container
  at java.awt.Container.checkNotAWindow(Container.java:483)
  at java.awt.Container.addImpl(Container.java:1084)
  at java.awt.Container.add(Container.java:966)
  at Lab2.EmployeeGUI.main(EmployeeGUI.java:28)
Excep