JTabbedPane.setComponentAt() should be allowed to replace the component with null. Currently, if you replace a valid component with null, you get a NPE:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
        at javax.swing.JTabbedPane.setComponentAt(JTabbedPane.java:1398)

The code tries to set visibility to true and add the component even when non-null.