Related documents plugins logs warnings about an 'Undetached JcrItemModel'

When the CMS is run in Wicket development mode, the related docs plugin logs many warnings while viewing/editing a document that contains a 'relateddocs' field.

Steps to reproduce:
1. start the GoGreen CMS with "mvn -Pcargo.run -Dcargo.jvm.args='-Dwicket.configuration=development'
2. login to the CMS as admin
3. open the document 'Hippo Go Green -> Events -> 2010 -> 12 -> NextGenFuels 2010'

Expected behavior: no warnings and logged
Actual behavior: many identical warnings like this are logged:

[INFO] [talledLocalContainer] 23.09.2011 11:30:44 WARN [org.hippoecm.frontend.model.JcrItemModel.writeObject():271] Undetached JcrItemModel /content/documents/hippogogreen/events/2010/12/nextgenfuels-2010/nextgenfuels-2010/relateddocs:docs