NullPointerException in AuthenticationEngine

When opening large numbers of tabs containing SP sites which attempt to login with PreviousSession handler, sometimes the flow is corrupted and strange errors are produced. A number of these we already discussed on the user list, but one of them is an bug. This is guaranteed to show up when opening many tabs at once. Rarely has it been seen so far in the production logs, but a few instances have appeared. Here is a stack trace:

Aug 8, 2011 7:23:00 AM org.apache.catalina.core.ApplicationDispatcher invoke
SEVERE: Servlet.service() for servlet AuthenticationEngine threw exception
java.lang.NullPointerException
at edu.internet2.middleware.shibboleth.idp.authn.AuthenticationEngine.returnToProfileHandler(AuthenticationEngine.java:168)
at edu.internet2.middleware.shibboleth.idp.authn.AuthenticationEngine.completeAuthentication(AuthenticationEngine.java:560)
at edu.internet2.middleware.shibboleth.idp.authn.AuthenticationEngine.service(AuthenticationEngine.java:218)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.apache.catalina.core.ApplicationDispatcher.invoke(ApplicationDispatcher.java:630)
at org.apache.catalina.core.ApplicationDispatcher.processRequest(ApplicationDispatcher.java:436)
at org.apache.catalina.core.ApplicationDispatcher.doForward(ApplicationDispatcher.java:374)
at org.apache.catalina.core.ApplicationDispatcher.forward(ApplicationDispatcher.java:302)
at edu.internet2.middleware.shibboleth.idp.authn.AuthenticationEngine.forwardRequest(AuthenticationEngine.java:189)
at edu.internet2.middleware.shibboleth.idp.authn.AuthenticationEngine.returnToAuthenticationEngine(AuthenticationEngine.java:150)
at edu.internet2.middleware.shibboleth.idp.authn.provider.PreviousSessionLoginHandler.login(PreviousSessionLoginHandler.java:116)
at edu.internet2.middleware.shibboleth.idp.authn.AuthenticationEngine.startUserAuthentication(AuthenticationEngine.java:258)
at edu.internet2.middleware.shibboleth.idp.authn.AuthenticationEngine.service(AuthenticationEngine.java:216)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at edu.internet2.middleware.shibboleth.idp.util.NoCacheFilter.doFilter(NoCacheFilter.java:50)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at edu.internet2.middleware.shibboleth.idp.session.IdPSessionFilter.doFilter(IdPSessionFilter.java:81)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at edu.internet2.middleware.shibboleth.common.log.SLF4JMDCCleanupFilter.doFilter(SLF4JMDCCleanupFilter.java:52)
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:219)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:128)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
at org.terracotta.modules.tomcat.tomcat_5_5.SessionValve55.invoke(SessionValve55.java:87)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:286)
at org.apache.jk.server.JkCoyoteHandler.invoke(JkCoyoteHandler.java:190)
at org.apache.jk.common.HandlerRequest.invoke(HandlerRequest.java:283)
at org.apache.jk.common.ChannelSocket.invoke(ChannelSocket.java:767)
at org.apache.jk.common.ChannelSocket.processConnection(ChannelSocket.java:697)
at org.apache.jk.common.ChannelSocket$SocketConnection.runIt(ChannelSocket.java:889)
at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:690)
at java.lang.Thread.run(Thread.java:662)