Fix to make things serializable

Change-Id: Id6ec282d72065241f8942c2b169311de5710a6b7
diff --git a/core/store/dist/src/main/java/org/onlab/onos/store/cluster/messaging/MessageSubject.java b/core/store/dist/src/main/java/org/onlab/onos/store/cluster/messaging/MessageSubject.java
index ee3e789..43df15f 100644
--- a/core/store/dist/src/main/java/org/onlab/onos/store/cluster/messaging/MessageSubject.java
+++ b/core/store/dist/src/main/java/org/onlab/onos/store/cluster/messaging/MessageSubject.java
@@ -45,4 +45,9 @@
         MessageSubject that = (MessageSubject) obj;
         return Objects.equals(this.value, that.value);
     }
+
+    // for serializer
+    protected MessageSubject() {
+        this.value = "";
+    }
 }