Configuration editor: Session hangs with OutOfBounds at cancel add new sitemap item

CMS session hangs with an OutOfBounds error.

1) In URL design I click the button 'add new sitemap item'
2) A child node named 'new' appears
3) A popup dialog appears
4) In the dialog I click 'cancel'
5) In the tree I click another node (an uncle, i.e. sibbling of parent).
6) The CMS seems to hang, with the following error:

ERROR [org.apache.wicket.RequestCycle.logRuntimeException():1437] Exception in rendering component: [MarkupContainer [Component id = link]]
org.apache.wicket.WicketRuntimeException: Exception in rendering component: [MarkupContainer [Component id = link]]
...
at org.mortbay.thread.QueuedThreadPool$PoolThread.run(QueuedThreadPool.java:520)
Caused by: java.lang.ArrayIndexOutOfBoundsException: -1
at java.util.ArrayList.get(ArrayList.java:323)

7) Although most clickable elements were not clickable anymore, deleting the wrong node was possible.
8) At leads once this lead to more out of bounds and clickable elements being not clickable anymore.