Bugfix for double PORT_STATS_UPDATED DeviceEvent

Change-Id: I5e53c3046fce23afcd8f22813bc2d81f95952e96
diff --git a/core/store/dist/src/main/java/org/onosproject/store/device/impl/GossipDeviceStore.java b/core/store/dist/src/main/java/org/onosproject/store/device/impl/GossipDeviceStore.java
index 7d0be64..a540e56 100644
--- a/core/store/dist/src/main/java/org/onosproject/store/device/impl/GossipDeviceStore.java
+++ b/core/store/dist/src/main/java/org/onosproject/store/device/impl/GossipDeviceStore.java
@@ -860,7 +860,8 @@
         }
         devicePortDeltaStats.put(deviceId, deltaStatsMap);
         devicePortStats.put(deviceId, newStatsMap);
-        return new DeviceEvent(PORT_STATS_UPDATED,  devices.get(deviceId), null);
+        // DeviceEvent returns null because of InternalPortStatsListener usage
+        return null;
     }
 
     /**