NullPointerException in ActiveMQSession.rollback()

Sometimes we get this exception:

java.lang.NullPointerException
at org.apache.activemq.ActiveMQSession.rollback(ActiveMQSession.java:581)
at com.barra.cp.common.activemq.MultiQueueReceiver.rollback(MultiQueueReceiver.java:212)
at com.barra.cp.common.activemq.MultiQueueReceiver.onMessage(MultiQueueReceiver.java:185)

The synchronized dispose() method nulls the transactionContext and sets closed to true. In many places in the code, not synchronized code checks closed member variable and later references transactionContext, which can become null by that time.