[AMQ7, scale down] ha shared store, slave broker throwing Nullpointer when configured to scale down

Issue

• ha shared store, slave broker configured to scale down, throwing the following NullPointer Exception when initialising

2018-09-03 20:45:12,970 ERROR [org.apache.activemq.artemis.core.server] AMQ224000: Failure in initialisation: java.lang.NullPointerException
    at org.apache.activemq.artemis.core.server.impl.BackupRecoveryJournalLoader.postLoad(BackupRecoveryJournalLoader.java:90) [artemis-server-2.6.1.amq-720008.jar:2.6.1.amq-720008]
    at org.apache.activemq.artemis.core.persistence.impl.journal.AbstractJournalStorageManager.loadMessageJournal(AbstractJournalStorageManager.java:1191) [artemis-server-2.6.1.amq-720008.jar:2.6.1.amq-720008]
    at org.apache.activemq.artemis.core.server.impl.ActiveMQServerImpl.loadJournals(ActiveMQServerImpl.java:2621) [artemis-server-2.6.1.amq-720008.jar:2.6.1.amq-720008]
    at org.apache.activemq.artemis.core.server.impl.ActiveMQServerImpl.initialisePart2(ActiveMQServerImpl.java:2376) [artemis-server-2.6.1.amq-720008.jar:2.6.1.amq-720008]
    at org.apache.activemq.artemis.core.server.impl.SharedStoreBackupActivation.run(SharedStoreBackupActivation.java:90) [artemis-server-2.6.1.amq-720008.jar:2.6.1.amq-720008]
    at org.apache.activemq.artemis.core.server.impl.ActiveMQServerImpl$ActivationThread.run(ActiveMQServerImpl.java:3127) [artemis-server-2.6.1.amq-720008.jar:2.6.1.amq-720008]