Recurrence of cms-7942: Warning logged when field is added in document type editor

This issue is created for the recurrence of the issue CMS7-7942
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Whenever a field is added to a document type the in the document type editor, a warning is logged.

Steps to reproduce:
1. login as admin to the archetype CMS
2. navigate to the Documents > Configuration tab
3. edit the 'newsdocument' type
4. add a string field

Expected: no warnings in the logs
Actual: the following warning is logged:

[INFO] [talledLocalContainer] 01.05.2014 12:53:54 WARN http-bio-8080-exec-4 [AbstractTextComponent.getModelType:170] Couldn't resolve model type of Model:classname=[org.apache.wicket.model.PropertyModel]:nestedModel=[Model:classname=[org.apache.wicket.model.Model]:object=[JcrPluginConfig:/hippo:namespaces/myhippoproject/newsdocument/editor:templates/_default_/string]]:expression=[hint] for [TextArea [Component id = widget]], please set the type yourself.