ArrayIndexOutOfBoundsException in TreePacksPanel

The TreePacksPanel
panel raises an ArrayIndexOutOfBoundsException if any of its children is
marked as hidden (see http://jira.codehaus.org/browse/IZPACK-461 which
seems to be back).

A stacktrace sample:
java.lang.IndexOutOfBoundsException: Index: 24, Size: 24
	at java.util.ArrayList.RangeCheck(ArrayList.java:547)
	at java.util.ArrayList.get(ArrayList.java:322)
	at
com.izforge.izpack.panels.packs.PacksModel.getValueAt(PacksModel.java:435)
	at
com.izforge.izpack.panels.treepacks.TreePacksPanel.updateModel(TreePacksPanel.java