In the JSP you have included the header fragment, but the URL used to get the resource isn't working because it contains an OGNL expression that unable to evaluate, because it references to the null
object.
Now I see you posted ho do you retrieve the URL
<s:property value="#mainViewComponent.contextPath" />
And error is that #mainViewComponent
referenced the ActionContext
property which is null
because you didn't put the object to it, but you put it in the valueStack
so should get
<s:property value="mainViewComponent.contextPath" />