The only thing I can think of is that somewhere in your code (
servlet, helper class, or
JSP scriptlets) you are doing a setAttribute(org.apache.struts.action.MESSAGE,
someValue), where someValue evaluates to null. If you aren't using that key directly, look for places where you might have assigned its value to some variable and use that to setAttribute() to null.
Or you could be clobbering it with a bean tag somewhere in your JSP. Again, look for org.apache.struts.action.MESSAGE or something that was assigned its value.
This is just a guess though and I may very well be out in left field on this...