Fixes NPE in ConsistentDeviceMastershipStore

Change-Id: Id07c8d41338a48e58d7e3ea98312691a2011eeef

@@ -236,7 +236,8 @@ public class ConsistentDeviceMastershipStore
 
         String leadershipTopic = createDeviceMastershipTopic(deviceId);
         Leadership leadership = leadershipService.getLeadership(leadershipTopic);
-        return leadership != null ? MastershipTerm.of(leadership.leaderNodeId(), leadership.leader().term()) : null;
+        return leadership != null && leadership.leaderNodeId() != null ?
+            MastershipTerm.of(leadership.leaderNodeId(), leadership.leader().term()) : null;
     }
 
     @Override
@@ -349,4 +350,4 @@ public class ConsistentDeviceMastershipStore
         Matcher m = DEVICE_MASTERSHIP_TOPIC_PATTERN.matcher(topic);
         return m.matches();
     }
-}
\ No newline at end of file
+}