In some JSP we access static fields (constants) of an interface. The interface has scope public and and the fields are public static final.
When accessing the value of such a field the ValueStack throws a NullPointerException here (com.opensymphony.xwork2.util.ProxyUtil), because there is actually no object (parameter holds null) when accessing a static member:
Support to access static members is switched on via struts constant
<constant name="struts.ognl.allowStaticMethodAccess" value="true" />