This is caused by updateClusterStatus rpcs returning null if a merged node has master status.
Make sure all rpcs get the same treatment.

2009-07-27 14:29:48,134 INFO [org.jboss.ha.framework.interfaces.HAPartition.DefaultPartition] (Incoming-20,10.248.107.114:32770) New cluster view for partition DefaultPartition: 4 ([10.240.62.47:1099, 10.248.107.114:1099, 10.249.195.196:1099] delta: 2)
2009-07-27 14:29:48,137 INFO [org.jboss.ha.framework.server.DistributedReplicantManagerImpl.DefaultPartition] (AsynchViewChangeHandler Thread) Merging partitions...
2009-07-27 14:29:48,137 INFO [org.jboss.ha.framework.server.DistributedReplicantManagerImpl.DefaultPartition] (AsynchViewChangeHandler Thread) Dead members: 0
2009-07-27 14:29:48,137 INFO [org.jboss.ha.framework.server.DistributedReplicantManagerImpl.DefaultPartition] (AsynchViewChangeHandler Thread) Originating groups: [[10.240.62.47:32769|3] [10.240.62.47:32769, 10.249.195.196:32769], [10.248.107.114:32770|0] [10.248.107.114:32770]]
2009-07-27 14:29:50,878 ERROR [org.apache.catalina.core.ContainerBase] (ContainerBackgroundProcessor[StandardEngine[jboss.web]]) Exception invoking periodic operation:
java.lang.NullPointerException
at org.jboss.modcluster.ha.HAModClusterService$ClusteredCatalinaEventHandler.updateClusterStatus(HAModClusterService.java:896)
at org.jboss.modcluster.ha.HAModClusterService$ClusteredCatalinaEventHandler.status(HAModClusterService.java:836)
at org.jboss.modcluster.ha.HAModClusterService$ClusteredCatalinaEventHandler.status(HAModClusterService.java:781)
at org.jboss.modcluster.CatalinaEventHandlerAdapter.lifecycleEvent(CatalinaEventHandlerAdapter.java:157)
at org.apache.catalina.util.LifecycleSupport.fireLifecycleEvent(LifecycleSupport.java:117)
at org.apache.catalina.core.ContainerBase.backgroundProcess(ContainerBase.java:1348)
at org.apache.catalina.core.ContainerBase$ContainerBackgroundProcessor.processChildren(ContainerBase.java:1612)
at org.apache.catalina.core.ContainerBase$ContainerBackgroundProcessor.run(ContainerBase.java:1601)
at java.lang.Thread.run(Thread.java:636)