[ARTEMIS-502] AMQ224044, error acknowledging message: java.lang.NullPointerException, can happen sometimes during load

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

@@ -930,6 +930,11 @@ public class ServerConsumerImpl implements ServerConsumer, ReadyListener {
 
       synchronized (lock) {
          // This is an optimization, if the reference is the first one, we just poll it.
+         // But first we need to make sure deliveringRefs isn't empty
+         if (deliveringRefs.isEmpty()) {
+            return null;
+         }
+
          if (deliveringRefs.peek().getMessage().getMessageID() == messageID) {
             return deliveringRefs.poll();
          }