Sporadic error with factory.requestHelper

• Application server:

  Tomcat 5.0

from displaytag forum:

Hi I'm getting the following error sporadically with Tomcat 5.0 and Struts
1.2
ERROR properties.TableProperties - Exception: [.TableProperties] Cannot instantiate
the class "null", specified by the property "factory.requestHelper".
Cause: null
java.lang.NullPointerException
       at java.lang.Class.forName0(Native Method)
       at java.lang.Class.forName(Class.java:141)
       at org.displaytag.properties.TableProperties.getClassPropertyInstance(TableProp
erties.java:785)
       at org.displaytag.properties.TableProperties.getRequestHelperFactoryInstance(Ta
bleProperties.java:728)
       at org.displaytag.tags.TableTag.initParameters(TableTag.java:669)
       at org.displaytag.tags.TableTag.doStartTag(TableTag.java:551)
       at org.apache.jsp.web.jsp.searchlist_jsp._jspService(searchlist_jsp.java:355)
       at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:94)
       at javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
       at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:3
24)
       at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:292)
       at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:236)
       at javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
       at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(Application
FilterChain.java:237)
       at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterCh
ain.java:157)
       at org.apache.catalina.core.ApplicationDispatcher.invoke(ApplicationDispatcher.
java:704)
       at org.apache.catalina.core.ApplicationDispatcher.processRequest(ApplicationDis
patcher.java:474)
       at org.apache.catalina.core.ApplicationDispatcher.doForward(ApplicationDispatch
er.java:409)
       at org.apache.catalina.core.ApplicationDispatcher.forward(ApplicationDispatcher
.java:312)
       at org.apache.struts.action.RequestProcessor.doForward(RequestProcessor.java:10
56)
       at org.apache.struts.action.RequestProcessor.processForwardConfig(RequestProces
sor.java:388)
       at org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:231)
       at org.apache.struts.action.ActionServlet.process(ActionServlet.java:1164)
       at org.apache.struts.action.ActionServlet.doPost(ActionServlet.java:415)
       at javax.servlet.http.HttpServlet.service(HttpServlet.java:709)
       at javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
       at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(Application
FilterChain.java:237)
       at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterCh
ain.java:157)
       at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.ja
va:214)
       at org.apache.catalina.core.StandardValveContext.invokeNext(StandardValveContex
t.java:104)
       at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:520)
       at org.apache.catalina.core.StandardContextValve.invokeInternal(StandardContext
Valve.java:198)
       at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.ja
va:152)
       at org.apache.catalina.core.StandardValveContext.invokeNext(StandardValveContex
t.java:104)
       at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:520)
       at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:137
)
       at org.apache.catalina.core.StandardValveContext.invokeNext(StandardValveContex
t.java:104)
       at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:118
)
       at org.apache.catalina.core.StandardValveContext.invokeNext(StandardValveContex
t.java:102)
       at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:520)
       at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java
:109)
       at org.apache.catalina.core.StandardValveContext.invokeNext(StandardValveContex
t.java:104)
       at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:520)
       at org.apache.catalina.core.ContainerBase.invoke(ContainerBase.java:929)
       at org.apache.coyote.tomcat5.CoyoteAdapter.service(CoyoteAdapter.java:160)
       at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:799)
       at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.processConne
ction(Http11Protocol.java:705)
       at org.apache.tomcat.util.net.TcpWorkerThread.runIt(PoolTcpEndpoint.java:577)
       at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.jav
a:683)
       at java.lang.Thread.run(Thread.java:534)