I have encountered this during writing more complete unit testing for sfsb clustering. The test case is:
org.jboss.ejb3.test.clusteredsession.unit.testStatefulBeanCounterFailover

Here is how to reproduce it:

Create a ParentBean and NestedBean for nested sfsb case. Hit node1, failover to node2, and execute will produce the following exception:

Caused by: java.lang.NullPointerException^M
at org.jboss.ejb3.stateful.StatefulBeanContextReference.<init>(StatefulBeanContextReference.java:51)^M
at org.jboss.ejb3.stateful.ProxiedStatefulBeanContext.writeExternal(ProxiedStatefulBeanContext.java:92)^M
at org.jboss.serial.persister.ExternalizePersister.writeData(ExternalizePersister.java:58)^M
at org.jboss.serial.objectmetamodel.ObjectDescriptorFactory.describeObject(ObjectDescriptorFactory.java:275)^M
at org.jboss.serial.objectmetamodel.DataContainer$DataContainerDirectOutput.writeObject(DataContainer.java:197)^M
at org.jboss.serial.io.JBossObjectOutputStreamSharedTree.writeObjectOverride(JBossObjectOutputStreamSharedTree.java:59)^M
at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:287)^M
at org.jboss.cache.marshall.EnhancedTreeCacheMarshaller.marshallObject(EnhancedTreeCacheMarshaller.java:258)^M
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Note that this error has already existed before any refactoring is in place.