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You are most likely trying to bind a property that returns a PersistentBag to a field that expects a String. For example, you are trying to bind a property of an item from a JPAContainer to a TextField, the property itself is not of type string.
Secondly, PersistentBag is a collection and a TextField cannot handle collections.
Check what property you are binding and where you are binding it to. Make sure that the property's type and the field's type matches. Otherwise you'll need to use a Converter.