Unable to authenticate in SPNEGO Login Module with NullPointerException

Description of problem:

The configuration with SPNEGO works fine, however from time to time the authentication fails with the following error:

ERROR (HTTP-341) [org.jboss.security.auth.spi.AbstractServerLoginModule] Unable to authenticate: java.lang.NullPointerException
at org.jboss.security.negotiation.spnego.SPNEGOLoginModule$AcceptSecContext.run(SPNEGOLoginModule.java:420)
at java.security.AccessController.doPrivileged(Native Method)
at javax.security.auth.Subject.doAs(Subject.java:356)

Version-Release number of selected component (if applicable):

JBoss Security Negotiation 2.3.3.Final

How reproducible:
This happens very rarely (20 times in a day on a system where about 50 users are working) and it is extremely hard to reproduce.

Additional info:

At line 420 in [1], the GSSToken is null
~~~~
if (respToken != null)
{
NegotiationMessage response;
if (requestMessage instanceof KerberosMessage)

{ response = new KerberosMessage(Constants.KERBEROS_V5, respToken); }

else

{ NegTokenTarg negTokenTarg = new NegTokenTarg(); negTokenTarg.setResponseToken(respToken); response = negTokenTarg; }

~~~~

It looks like a GSSToken can be or is null, check the line#344 as follows:-
~~~~~~~~~
public Object run()
{
try
{
// The message type will have already been checked before this point so we know it is
// a SPNEGO message.
NegotiationMessage requestMessage = negotiationContext.getRequestMessage();

// TODO - Ensure no way to fall through with gssToken still null.
byte[] gssToken = null;
if (requestMessage instanceof NegTokenInit)
{
...
~~~~~~~~~

[1] : https://github.com/wildfly-security/jboss-negotiation/blob/2.3.3.Final/jboss-negotiation-spnego/src/main/java/org/jboss/security/negotiation/spnego/SPNEGOLoginModule.java