Error

java.lang.NullPointerException
   at java.util.Hashtable.get(Hashtable.java:363)
   at org.histpop.ohpr.session.CategoryState.findNode(CategoryState.java:81)
   at org.histpop.ohpr.session.CategoryState.findNode(CategoryState.java:92)
   at org.histpop.ohpr.frames.CategoryFrame.updateCategoryState(CategoryFrame.java:255)
   at org.histpop.ohpr.frames.CategoryFrame.display(CategoryFrame.java:101)
   at org.histpop.server.TestServlet.displayResult(TestServlet.java:375)
   at org.histpop.ohpr.OHPRServlet.displayResult(OHPRServlet.java:423)
   at org.histpop.server.TestServlet.executeRequest(TestServlet.java:241)
   at org.histpop.ohpr.OHPRServlet.executeRequest(OHPRServlet.java:175)
   at org.histpop.server.TestServlet.processRequest(TestServlet.java:176)
   at org.histpop.ohpr.OHPRServlet.processRequest(OHPRServlet.java:131)
   at org.histpop.server.TestServlet.doGet(TestServlet.java:137)
   at javax.servlet.http.HttpServlet.service(HttpServlet.java:624)
   at javax.servlet.http.HttpServlet.service(HttpServlet.java:731)
   at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303)
   at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
   at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
   at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
   at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
   at org.apache.catalina.filters.RequestFilter.process(RequestFilter.java:206)
   at org.apache.catalina.filters.RemoteAddrFilter.doFilter(RemoteAddrFilter.java:71)
   at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
   at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
   at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:218)
   at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:110)
   at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:506)
   at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:169)
   at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:103)
   at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:962)
   at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116)
   at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:445)
   at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1115)
   at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:637)
   at org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.doRun(AprEndpoint.java:2549)
   at org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.run(AprEndpoint.java:2538)
   at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
   at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
   at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
   at java.lang.Thread.run(Thread.java:748)