Kryo related changes.

Change-Id: I5b4fab63d6ece084b65aa712971a22d953d0caf0
diff --git a/core/api/src/main/java/org/onlab/onos/net/ElementId.java b/core/api/src/main/java/org/onlab/onos/net/ElementId.java
index e205bb6..a75f11e 100644
--- a/core/api/src/main/java/org/onlab/onos/net/ElementId.java
+++ b/core/api/src/main/java/org/onlab/onos/net/ElementId.java
@@ -10,6 +10,11 @@
 
     private final URI uri;
 
+    // Default constructor for serialization
+    protected ElementId() {
+        this.uri = null;
+    }
+
     /**
      * Creates an element identifier using the supplied URI.
      *