Erro no console ao executar relatório no Struts 2

Programação Java
#1

Ola Pessoal,

Estou fazendo o relatório em excel com o Struts 2 e está ebibindo perfeito o relatório, mas no console dá o seguinte erro:

java.lang.IllegalStateException
        at org.apache.catalina.connector.ResponseFacade.sendError(ResponseFacade.java:486)
        at org.apache.struts2.dispatcher.Dispatcher.sendError(Dispatcher.java:770)
        at org.apache.struts2.dispatcher.Dispatcher.serviceAction(Dispatcher.java:505)
        at org.apache.struts2.dispatcher.ng.ExecuteOperations.executeAction(ExecuteOperations.java:77)
        at org.apache.struts2.dispatcher.ng.filter.StrutsPrepareAndExecuteFilter.doFilter(StrutsPrepareAndExecuteFilter.java:91)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:256)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:215)
        at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:277)
        at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:188)
        at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:641)
        at com.sun.enterprise.web.WebPipeline.invoke(WebPipeline.java:97)
        at com.sun.enterprise.web.PESessionLockingStandardPipeline.invoke(PESessionLockingStandardPipeline.java:85)
        at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:185)
        at org.apache.catalina.connector.CoyoteAdapter.doService(CoyoteAdapter.java:325)
        at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:226)
        at com.sun.enterprise.v3.services.impl.ContainerMapper.service(ContainerMapper.java:165)
        at com.sun.grizzly.http.ProcessorTask.invokeAdapter(ProcessorTask.java:791)
        at com.sun.grizzly.http.ProcessorTask.doProcess(ProcessorTask.java:693)
        at com.sun.grizzly.http.ProcessorTask.process(ProcessorTask.java:954)
        at com.sun.grizzly.http.DefaultProtocolFilter.execute(DefaultProtocolFilter.java:170)
        at com.sun.grizzly.DefaultProtocolChain.executeProtocolFilter(DefaultProtocolChain.java:135)
        at com.sun.grizzly.DefaultProtocolChain.execute(DefaultProtocolChain.java:102)
        at com.sun.grizzly.DefaultProtocolChain.execute(DefaultProtocolChain.java:88)
        at com.sun.grizzly.http.HttpProtocolChain.execute(HttpProtocolChain.java:76)
        at com.sun.grizzly.ProtocolChainContextTask.doCall(ProtocolChainContextTask.java:53)
        at com.sun.grizzly.SelectionKeyContextTask.call(SelectionKeyContextTask.java:57)
        at com.sun.grizzly.ContextTask.run(ContextTask.java:69)
        at com.sun.grizzly.util.AbstractThreadPool$Worker.doWork(AbstractThreadPool.java:330)
        at com.sun.grizzly.util.AbstractThreadPool$Worker.run(AbstractThreadPool.java:309)
        at java.lang.Thread.run(Thread.java:619)

A classe Action é:

import br.com.relatorio.Relatorio_Excel;
import br.com.bean.Aluno;
import com.opensymphony.xwork2.ActionContext;
import java.util.ArrayList;
import javax.servlet.ServletOutputStream;
import javax.servlet.http.HttpServletResponse;
import org.apache.poi.hssf.usermodel.HSSFWorkbook;
import org.apache.struts2.convention.annotation.Action;
import org.apache.struts2.convention.annotation.Result;

public class DownloadAction {

    @Action(value = "download", results = {
        @Result(name = "success", type = "stream", location = "/index.jsp")
    })
    public String download() throws Exception {

        Aluno aluno = new Aluno();

        ArrayList<Aluno> listaAluno = new ArrayList<Aluno>();

        //Preenche a lista de aluno, e joga na lista de aluno
        for (int i = 0; i <= 10; i++) {
            aluno.setNome("Nome" + i);
            aluno.setNome("Idade" + i);

            //Adiciono a lista de aluno, obitida do for
            listaAluno.add(aluno);
        }

        Relatorio_Excel relatorioExcel = new Relatorio_Excel();

        //Nome da aba do excel que será gerada
        //Monta o relatório em Excel
        relatorioExcel.relatorioExcelExemplo("Cadastro de Aluno", listaAluno);
        HSSFWorkbook wb = relatorioExcel.getWb();        
        
        HttpServletResponse response  = (HttpServletResponse) ActionContext.getContext().get(org.apache.struts2.StrutsStatics.HTTP_RESPONSE);
        response.setContentType("text/plain");  
        response.setHeader("Content-Disposition", "attachment; filename=\"Cadastro Aluno.xls\"");
        response.setHeader("Cache-Control", "max-age=0");

        ServletOutputStream out = response.getOutputStream();
        wb.write(out);
        out.flush();
	out.close();
        
        return "success";
    }
}

Alguém sabe porque dá esse erro?

Obrigado.

#2

Bota um try-catch nesse método download pra pegar o erro direitinho.

#3