有没有人给我解释一下这个错误呀？

HTTP Status 500 - --------------------------------------------------------------------------------type Exception reportmessage description The server encountered an internal error () that prevented it from fulfilling this request.exception javax.servlet.ServletException
org.apache.struts.action.RequestProcessor.processException(RequestProcessor.java:526)
org.apache.struts.action.RequestProcessor.processActionPerform(RequestProcessor.java:467)
org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:274)
org.apache.struts.action.ActionServlet.process(ActionServlet.java:1422)
org.apache.struts.action.ActionServlet.doPost(ActionServlet.java:523)
javax.servlet.http.HttpServlet.service(HttpServlet.java:709)
javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
root cause java.lang.NullPointerException
logon.LogonAction.execute(LogonAction.java:253)
org.apache.struts.action.RequestProcessor.processActionPerform(RequestProcessor.java:465)
org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:274)
org.apache.struts.action.ActionServlet.process(ActionServlet.java:1422)
org.apache.struts.action.ActionServlet.doPost(ActionServlet.java:523)
javax.servlet.http.HttpServlet.service(HttpServlet.java:709)
javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
note The full stack trace of the root cause is available in the Apache Tomcat/5.0.28 logs.