I reported this as http://jira.opensymphony.com/browse/XW-737 and I was asked to report this here as well.
Upon upgrading to struts 2.1.8 (from 2.0.14) with xwork 2.1.6, I started getting field errors which included the string null in them for field visitor validators.
Using a visitor validator as follows:
and a required string validator on the actual field as follows:
<message>Description is required</message>
I was expecting the field for residenceRoom.description to contain "Description is required" but instead it contained "nullDescription is required". Presumably this happens because of line 202 in VisitorFieldValidator.java
super.addFieldError(getFullFieldName(fieldName), message + errorMessage);
The fix for me seems to be to fix ValidatorSupport to not nullify defaultMessage when message is null or empty. I will attach a patch shortly.