Foro de Struts

me sale  este error, apesar que ingreso datos

excepción

javax.servlet.ServletException: null
org.apache.struts.action.RequestProcessor.processException(RequestProcessor.java:545)
org.apache.struts.action.RequestProcessor.processActionPerform(RequestProcessor.java:486)
org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:274)
org.apache.struts.action.ActionServlet.process(ActionServlet.java:1482)
org.apache.struts.action.ActionServlet.doGet(ActionServlet.java:507)
javax.servlet.http.HttpServlet.service(HttpServlet.java:689)
javax.servlet.http.HttpServlet.service(HttpServlet.java:802)


causa raíz 

java.lang.NumberFormatException: null
java.lang.Integer.parseInt(Unknown Source)
java.lang.Integer.parseInt(Unknown Source)
lp3.action.preguntaAction.guarda(preguntaAction.java:93)
lp3.action.preguntaAction.execute(preguntaAction.java:40)
org.apache.struts.action.RequestProcessor.processActionPerform(RequestProcessor.java:484)
org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:274)
org.apache.struts.action.ActionServlet.process(ActionServlet.java:1482)
org.apache.struts.action.ActionServlet.doGet(ActionServlet.java:507)
javax.servlet.http.HttpServlet.service(HttpServlet.java:689)
javax.servlet.http.HttpServlet.service(HttpServlet.java:802)