NullPointerException during failback - EAP/HornetQ in collocated/dedicated topology

There is NullPointerException during failback.

Tests scenario:

1. Start EAP with HQ in dedicated topology - server A, server A backup, server B, server B backup
2. Start producer and consumer for "queue/InQueue" - send 10000 messages
3. During step 2. kill server A -> clients failover to server A backup
4. Start server A again and wait until failback occurs
5. Kill server A backup

During step 4. when server A is trying to start - npe is thrown on server A backup

I was able to replicate the issue only in messaging lab with using SAN for journal. Still only 1 of 5 runs hit the issue.

Log from:
https://hudson.qa.jboss.com/hudson/view/EAP5/view/HornetQ-Perf/job/eap51-hornetq-failback-dedicated-jms-client/41/console-messaging-21/
npe:
14:07:52,638 WARN [ServerSessionPacketHandler] Sending unexpected exception to the client
[JBoss] java.lang.NullPointerException
[JBoss] at org.hornetq.core.server.impl.ServerSessionImpl.acknowledge(ServerSessionImpl.java:583)
[JBoss] at org.hornetq.core.protocol.core.ServerSessionPacketHandler.handlePacket(ServerSessionPacketHandler.java:270)
[JBoss] at org.hornetq.core.protocol.core.impl.ChannelImpl.handlePacket(ChannelImpl.java:504)
[JBoss] at org.hornetq.core.protocol.core.impl.RemotingConnectionImpl.doBufferReceived(RemotingConnectionImpl.java:538)
[JBoss] at org.hornetq.core.protocol.core.impl.RemotingConnectionImpl.bufferReceived(RemotingConnectionImpl.java:499)
[JBoss] at org.hornetq.core.remoting.server.impl.RemotingServiceImpl$DelegatingBufferHandler.bufferReceived(RemotingServiceImpl.java:512)
[JBoss] at org.hornetq.core.remoting.impl.netty.HornetQChannelHandler.messageReceived(HornetQChannelHandler.java:73)
[JBoss] at org.jboss.netty.channel.SimpleChannelHandler.handleUpstream(SimpleChannelHandler.java:100)
[JBoss] at org.jboss.netty.channel.StaticChannelPipeline.sendUpstream(StaticChannelPipeline.java:362)
[JBoss] at org.jboss.netty.channel.StaticChannelPipeline$StaticChannelHandlerContext.sendUpstream(StaticChannelPipeline.java:514)