java.lang.NullPointerException
	at org.zkoss.poi.xssf.model.CommentsTable.findAuthor(CommentsTable.java:103)
	at org.zkoss.poi.xssf.usermodel.XSSFComment.setAuthor(XSSFComment.java:65)
	at org.zkoss.zss.range.impl.imexp.AbstractExcelExporter.exportCell(AbstractExcelExporter.java:471)
	at org.zkoss.zss.range.impl.imexp.AbstractExcelExporter.exportRow(AbstractExcelExporter.java:367)
	at org.zkoss.zss.range.impl.imexp.AbstractExcelExporter.exportRowColumn(AbstractExcelExporter.java:329)
	at org.zkoss.zss.range.impl.imexp.AbstractExcelExporter.export(AbstractExcelExporter.java:131)
	at org.zkoss.zss.api.impl.ExporterImpl.export(ExporterImpl.java:49)
	at org.zkoss.zss.app.repository.impl.SimpleRepository.exportBook(SimpleRepository.java:136)
	at org.zkoss.zss.app.repository.impl.SimpleRepository.save(SimpleRepository.java:91)
	at org.zkoss.zss.app.repository.impl.SimpleRepository.saveAs(SimpleRepository.java:157)
	at org.zkoss.zssex.app.impl.BookManagerEx.saveBook(BookManagerEx.java:83)
	at org.zkoss.zss.app.ui.AppCtrl$17.onEvent(AppCtrl.java:627)
	at org.zkoss.zss.app.ui.AppCtrl$17.onEvent(AppCtrl.java:615)
	at org.zkoss.zss.app.ui.dlg.DlgCtrlBase$1.onEvent(DlgCtrlBase.java:57)
	at org.zkoss.zk.ui.AbstractComponent.onEvent(AbstractComponent.java:2752)
	at org.zkoss.zk.ui.AbstractComponent.service(AbstractComponent.java:2723)
	at org.zkoss.zk.ui.AbstractComponent.service(AbstractComponent.java:2664)
	at org.zkoss.zk.ui.impl.EventProcessor.process(EventProcessor.java:136)
	at org.zkoss.zk.ui.impl.UiEngineImpl.processEvent(UiEngineImpl.java:1765)
	at org.zkoss.zk.ui.impl.UiEngineImpl.process(UiEngineImpl.java:1550)
	at org.zkoss.zk.ui.impl.UiEngineImpl.execUpdate(UiEngineImpl.java:1260)
	at org.zkoss.zk.au.http.DHtmlUpdateServlet.process(DHtmlUpdateServlet.java:611)
	at org.zkoss.zk.au.http.DHtmlUpdateServlet.doGet(DHtmlUpdateServlet.java:485)
	at org.zkoss.zk.au.http.DHtmlUpdateServlet.doPost(DHtmlUpdateServlet.java:494)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:641)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:722)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:305)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
	at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:224)
	at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:169)
	at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:472)
	at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:168)
	at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:98)
	at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:927)
	at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:118)
	at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:407)
	at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:987)
	at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:579)
	at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:309)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1146)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
	at java.lang.Thread.run(Thread.java:701)