我用removeAllItems()想在没次选择省份时把市和县给清空，然后在下面在加选项，课每次我点击省份按钮就会报空指针，请问为什么？谢谢
然后出现了这个错误 ：Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException: Cannot invoke "String.equals(Object)" because the return value of "javax.swing.JComboBox.getSelectedItem()" is null
