ClearCacheInterceptor в struts2

В моем веб-приложении я хочу включить ClearCacheInterceptor, чтобы после выполнения действия выхода из системы он очищал улов, чтобы, если пользователь нажимал кнопку «Назад», он не отображал предыдущую страницу. когда я запускаю код, он работает нормально, я могу войти в систему, выполнить другое действие, но когда я нажимаю выход, он показывает ошибку. вот мой код

public class ClearCacheInterceptor  extends AbstractInterceptor{

/**
 *
 */
private static final long serialVersionUID = 1L;

@Override

public String intercept(ActionInvocation invocation) throws Exception {

    ActionContext context=(ActionContext)invocation.getInvocationContext();

    HttpServletResponse response=(HttpServletResponse)context.get(StrutsStatics.HTTP_RESPONSE);

    response.setHeader("Cache-Control", "no-cache");

    response.setHeader("Pragma", "no-cache");

    response.setDateHeader("Expires", 0);

    String result=invocation.invoke();

    System.out.println("check result="+result);

    return result;

}

}

код выхода

  public String logOut() {
    String status = "";
     if (true) {
        session.clear();
        session = null;
        if (session instanceof org.apache.struts2.dispatcher.SessionMap) {
            try {
                ((org.apache.struts2.dispatcher.SessionMap) session).invalidate();
            } catch (IllegalStateException e) {
            }
        }
        status = "success";
    } else {
        addActionError(getText("system.ERROR"));
        status = "error";
    }

    return status;
}  

мой файл действий struts.xml

       <struts>
<!-- Configuration for the default package. -->
<constant name="struts.Devmode" value="false"></constant>
<constant name="struts.enable.DynamicMethodInvocation" value="false" />
<constant name="struts.custom.i18n.resources" value="ApplicationResources" />
<package name="default" extends="struts-default">
    <interceptors>
        <interceptor-stack name="defaultStack">
            <interceptor-ref name="exception" />
            <interceptor-ref name="alias" />
            <interceptor-ref name="servletConfig" />
            <interceptor-ref name="prepare" />
            <interceptor-ref name="i18n" />
            <interceptor-ref name="chain" />
            <interceptor-ref name="debugging" />
            <interceptor-ref name="profiling" />
            <interceptor-ref name="scopedModelDriven" />
            <interceptor-ref name="modelDriven" />
            <interceptor-ref name="fileUpload" />
            <interceptor-ref name="checkbox" />
            <interceptor-ref name="staticParams" />
            <interceptor-ref name="params">
                <param name="excludeParams">dojo..*,d-..*,6..*</param>
            </interceptor-ref>
            <interceptor-ref name="conversionError" />
            <interceptor-ref name="validation">
                <param name="excludeMethods">input,back,cancel,browse</param>
            </interceptor-ref>
            <interceptor-ref name="workflow">
                <param name="excludeMethods">input,back,cancel,browse</param>
            </interceptor-ref>
        </interceptor-stack>
        <interceptor name="clear-cache"  class="com.java.session.ClearCacheInterceptor" />
    </interceptors>
    <default-interceptor-ref name="defaultStack"/>

    <action name="users">
        <result>home.jsp </result>
    </action>
    <action name="UserAction" class="com.java.action.RegisterAction">
        <result name="input">/signup.jsp</result>
        <result name="success">/home.jsp</result>
        <result name="error">/error.jsp</result>
    </action>
    <action name="trialregister">
        <result>signup.jsp</result>
    </action>
    <action name="Activate" class="com.java.action.ActivationAction">
        <result name="input">/error.jsp</result>
        <result name="success">/home.jsp</result>
        <result name="error">/test1.jsp</result>
    </action>
    <action name="verifyLogin" class="com.java.action.LoginAction" method="verifyLogin">
         <result name="input">/home.jsp</result>
        <result name="success">/test1.jsp</result>
    </action>

    <action name="logOut" class="com.java.action.LoginAction" method="logOut">
        <interceptor-ref name="clear-cache"/>
        <result name="error">/error.jsp</result>
        <result name="success">/logout.jsp</result>
    </action>

    <action name="Back">
    <result>/index.jsp</result>    
    </action>

    <action name="Reroot">
    <result>/index.jsp</result>    
    </action>

</package>

and error message in log is

    SEVERE: Servlet.service() for servlet default threw exception
          java.lang.NullPointerException
at com.java.action.LoginAction.logOut(LoginAction.java:103)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:601)
at com.opensymphony.xwork2.DefaultActionInvocation.invokeAction(DefaultActionInvocation.java:404)
at com.opensymphony.xwork2.DefaultActionInvocation.invokeActionOnly(DefaultActionInvocation.java:267)
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:229)
at com.java.session.ClearCacheInterceptor.intercept(ClearCacheInterceptor.java:37)
at com.opensymphony.xwork2.DefaultActionInvocation$2.doProfiling(DefaultActionInvocation.java:224)
at com.opensymphony.xwork2.DefaultActionInvocation$2.doProfiling(DefaultActionInvocation.java:223)
at com.opensymphony.xwork2.util.profiling.UtilTimerStack.profile(UtilTimerStack.java:455)
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:221)
at org.apache.struts2.impl.StrutsActionProxy.execute(StrutsActionProxy.java:50)
at org.apache.struts2.dispatcher.Dispatcher.serviceAction(Dispatcher.java:504)
at org.apache.struts2.dispatcher.FilterDispatcher.doFilter(FilterDispatcher.java:419)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:298)
at org.apache.coyote.http11.Http11AprProcessor.process(Http11AprProcessor.java:864)
at org.apache.coyote.http11.Http11AprProtocol$Http11ConnectionHandler.process(Http11AprProtocol.java:579)
at org.apache.tomcat.util.net.AprEndpoint$Worker.run(AprEndpoint.java:1665)
at java.lang.Thread.run(Thread.java:722)