Fix NPE in ADTabpanel while selecting a tree item

While studying error logs, I found this one :
NullPointerException: Cannot invoke "org.zkoss.zul.Treeitem.getValue()" because "item" is null
at org.adempiere.webui.adwindow.ADTabpanel.onEvent(ADTabpanel.java:1541)

Don't know how to reproduce it, but the fix should be enough