Fix to make things serializable

Change-Id: Id6ec282d72065241f8942c2b169311de5710a6b7
diff --git a/core/store/dist/src/main/java/org/onlab/onos/store/device/impl/InternalPortEvent.java b/core/store/dist/src/main/java/org/onlab/onos/store/device/impl/InternalPortEvent.java
index 48e3be6..64e77ca 100644
--- a/core/store/dist/src/main/java/org/onlab/onos/store/device/impl/InternalPortEvent.java
+++ b/core/store/dist/src/main/java/org/onlab/onos/store/device/impl/InternalPortEvent.java
@@ -37,4 +37,11 @@
     public Timestamped<List<PortDescription>> portDescriptions() {
         return portDescriptions;
     }
+
+    // for serializer
+    protected InternalPortEvent() {
+        this.providerId = null;
+        this.deviceId = null;
+        this.portDescriptions = null;
+    }
 }