I got the following stack trace when attempting to invalidate the session.

{{Caused by: java.lang.NullPointerException
at io.undertow.server.session.InMemorySessionManager$SessionImpl.requestStarted(InMemorySessionManager.java:459)
at io.undertow.server.session.InMemorySessionManager.getSession(InMemorySessionManager.java:218)
at org.wildfly.clustering.web.undertow.sso.DistributableSingleSignOn$InvalidatableSession.invalidate(DistributableSingleSignOn.java:182)
at io.undertow.security.impl.SingleSignOnAuthenticationMechanism$SessionInvalidationListener.sessionDestroyed(SingleSignOnAuthenticationMechanism.java:207)
at io.undertow.server.session.SessionListeners.sessionDestroyed(SessionListeners.java:61)
at io.undertow.server.session.InMemorySessionManager$SessionImpl.invalidate(InMemorySessionManager.java:586)

at io.undertow.server.session.InMemorySessionManager$SessionImpl.invalidate(InMemorySessionManager.java:560)

at io.undertow.servlet.spec.HttpSessionImpl.invalidate(HttpSessionImpl.java:198)}}

Looking at the InMemorySessionManager.java, the getSession or requestStarted could be improved with a check for a null parameter.