Steps to reproduce

1. run the project https://github.com/hawkchen/zssperformance on Tomcat 7
2. use Jmeter 2.10 to open test plan CollaborationEditing.jmx (the file is under [PROJECT-BASE]\TestPlan)(original test plan https://docs.google.com/a/potix.com/document/d/1pK-mi4hO14mKkmlf1qZLwi-wp3uU04wcMVzHk6MuS5I/edit#heading=h.i48nii91fa3p)
3. set number of thread to 50 in "Thread Group" and run the test plan
   

Expected

The tomcat 7 console shows no error.

What I actually see

The tomcat 7 console shows many NullPointerException.

Jun 24, 2014 2:50:02 PM org.zkoss.zk.ui.impl.UiEngineImpl handleError
SEVERE:
java.lang.NullPointerException
at org.zkoss.zss.ui.Spreadsheet.updateCell(Spreadsheet.java:2712)
at org.zkoss.zss.ui.Spreadsheet.access$4000(Spreadsheet.java:213)
at org.zkoss.zss.ui.Spreadsheet$InnerModelEventDispatcher.onCellContentChange(Spreadsheet.java:2346)
at org.zkoss.zss.ui.Spreadsheet$InnerModelEventDispatcher.access$1200(Spreadsheet.java:2058)
at org.zkoss.zss.ui.Spreadsheet$InnerModelEventDispatcher$7.onEvent(Spreadsheet.java:2102)
at org.zkoss.zss.ui.ModelEventDispatcher.onEvent(ModelEventDispatcher.java:49)
at org.zkoss.zss.model.impl.EventQueueListenerAdaptor$WrappedListener.onEvent(EventQueueListenerAdaptor.java:105)
at org.zkoss.zk.ui.event.impl.DesktopEventQueue$QueueListener.onEvent(DesktopEventQueue.java:152)
at org.zkoss.zk.ui.AbstractComponent.onEvent(AbstractComponent.java:2746)
at org.zkoss.zk.ui.AbstractComponent.service(AbstractComponent.java:2717)
at org.zkoss.zk.ui.AbstractComponent.service(AbstractComponent.java:2658)
at org.zkoss.zk.ui.impl.EventProcessor.process(EventProcessor.java:136)
at org.zkoss.zk.ui.impl.UiEngineImpl.processEvent(UiEngineImpl.java:1765)
at org.zkoss.zk.ui.impl.UiEngineImpl.process(UiEngineImpl.java:1550)
at org.zkoss.zk.ui.impl.UiEngineImpl.execUpdate(UiEngineImpl.java:1260)
at org.zkoss.zk.au.http.DHtmlUpdateServlet.process(DHtmlUpdateServlet.java:603)
at org.zkoss.zk.au.http.DHtmlUpdateServlet.doGet(DHtmlUpdateServlet.java:485)
at org.zkoss.zk.au.http.DHtmlUpdateServlet.doPost(DHtmlUpdateServlet.java:494)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:646)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:220)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:122)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:501)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:171)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:950)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:408)
at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1040)
at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:607)
at org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.doRun(AprEndpoint.java:2442)
at org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.run(AprEndpoint.java:2431)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
at java.lang.Thread.run(Thread.java:662)