[ARTEMIS-2050] It is possible to get AMQ224000: Failure in initialisa…

…tion: java.lang.NullPointerException during shutdown of backup server with shared store

(cherry picked from commit 3c3c817)

artemis-server/src/main/java/org/apache/activemq/artemis/core/server/impl/SharedStoreBackupActivation.java

@@ -114,7 +114,7 @@ public void run() {
 
             activeMQServer.getNodeManager().releaseBackup();
          }
-         if (sharedStoreSlavePolicy.isAllowAutoFailBack()) {
+         if (sharedStoreSlavePolicy.isAllowAutoFailBack() && ActiveMQServerImpl.SERVER_STATE.STOPPING != activeMQServer.getState() && ActiveMQServerImpl.SERVER_STATE.STOPPED != activeMQServer.getState()) {
             startFailbackChecker();
          }
       } catch (ClosedChannelException | InterruptedException e) {