NullPointerException in Struts 2

i stuck some strange bug today. somewhat wierd error message showed when i tried to login with unauthorized account. it’s only a 403 situation. why i have to see this error messages?

java.lang.NullPointerException
org.apache.struts2.views.jsp.TagUtils.getStack(TagUtils.java:58)
org.apache.struts2.views.jsp.StrutsBodyTagSupport.getStack(StrutsBodyTagSupport .java:54)
org.apache.struts2.views.jsp.ComponentTagSupport.doStartTag(ComponentTagSupport .java:49)
org.apache.jsp.WebContent.index_jsp._jspx_meth_s_005fhead_005f0(index_jsp.java: 155)
org.apache.jsp.WebContent.index_jsp._jspService(index_jsp.java:77)
org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:98)
javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:328)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:315)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:265)
javax.servlet.http.HttpServlet.service(HttpServlet.java:803)

at first, i thought it’s the problem of authentification. but that wasn’t. it’s because i use struts 2’s <s:if tag in my header.jsp. struts 2’s tags cannot be used in jsp directly. you have to use those with some actions. so my 403.jsp generates an error message for that. i change it to <c: and it works fine now.