AccessFilter issues in Struts

[Reshmi Kuttappan]

Hi,

i have been using the below code since some time now; but have started facing NullPointerException for the below scenario

my problem is:
------------------------
when my user clicks on the Back button instead of going to the previously visited page; it throws an error "HTTP Status 500 -
exception - javax.servlet.ServletException: BeanUtils.populate --> org.apache.struts.util.RequestUtils.populate(RequestUtils.java:1254)
root cause - java.lang.NullPointerException --> org.apache.commons.beanutils.PropertyUtilsBean.getIndexedProperty(PropertyUtilsBean.java:427)"

my log file provides following info:
---------------------------------------------
2012-06-05 16:42:36,442 DEBUG [org.displaytag.filter.ResponseOverrideFilter] Filter has been called, but PARAMETER_EXPORTING parameter has not been found.
2012-06-05 16:42:36,442 ERROR [org.apache.catalina.core.ContainerBase.[jboss.web].[localhost].[/torch].[action]] Servlet.service() for servlet action threw exception
java.lang.NullPointerException

when i debugged my java code; the error occurs in the AccessFilter.doFilter() method (Line number 51); since i am not able to debug the chain.doFilter() method; i don't know how to proceed ahead.
below is the code snippet for the same:

public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)throws  java.io.IOException, ServletException 
    {
    	HttpServletRequest  req = (HttpServletRequest)request;
    	HttpServletResponse res = (HttpServletResponse)response;
    	String requestedURL=req.getRequestURL().toString();
       	String  requestedAction=null;
    	StringTokenizer st =new StringTokenizer(requestedURL,"/");
    	boolean flag;
    	SecurityHelper securityHelper=null;
    	boolean isSkipAction=false;
    	boolean isLoggedInUser=false;
    	HttpSession session = null;
        
        //Set the character encoding for all the requests for multilingual handling
        log.debug("default encoding " + request.getCharacterEncoding());
        request.setCharacterEncoding(encoding);
    	while(st.hasMoreElements()){
    		requestedAction=st.nextToken();
    	}
    	
    	log.debug("Requested Action  value = " + requestedAction);
    	log.debug("altered action name " + requestedAction.substring(0,requestedAction.indexOf(".do")));
    	isSkipAction =skipAction(requestedAction);
    	requestedAction = requestedAction.substring(0,requestedAction.indexOf(".do"));
    	request.setAttribute(TorchConstants.CURRENT_ACTION,requestedAction);
    	log.debug("** isSkipAction -> "+isSkipAction);
       	if(!isSkipAction) {
    		chain.doFilter(request, response);
    	}
	   	
       	if(isSkipAction) {
       		isLoggedInUser=isLoggedIn(req);
	     	if(isLoggedInUser){ 	
				 log.debug("UserVO is not   null " + userVO.getSsoID());
				 securityHelper=new SecurityHelper();
				 flag = securityHelper.isAuthorized(userVO,requestedAction);
				 if(flag)
				 {
					 flag = isValidUser(req);
				 }
				 if(!flag)
				 {
					 log.debug("Access denied for request " + requestedAction);					 
				 }
				 
				 //Set the current action in user vo
		    	 userVO.setCurrentAction(requestedAction);
		    	 setSessionObject(req,AppConstants.GLOBAL_USERINFO_KEY, userVO);				 
				 log.debug("Flag = "+ flag);
				 if(flag || AppConstants.LOGIN_ACTION.equals(requestedAction)){
					 chain.doFilter(request, response);		     }else{
			    	 res.sendRedirect(TorchConstants.App_ACCESS_DENIED_PAGE);
			     }
			}
	     	else {
	     	  session = req.getSession(false);
			  //Invalidate the existing session.	
			  if (null != session) {
				 session.invalidate();
              }
	     	  res.sendRedirect(AppConstants.APP_SESSION_EXPIRE_PAGE);
			}
	    }	
    	
    }

please let me know how to proceed ahead with this.