Added private default constructors for serialization.
diff --git a/core/api/src/main/java/org/onlab/onos/net/AbstractElement.java b/core/api/src/main/java/org/onlab/onos/net/AbstractElement.java
index 01341a5..e810930 100644
--- a/core/api/src/main/java/org/onlab/onos/net/AbstractElement.java
+++ b/core/api/src/main/java/org/onlab/onos/net/AbstractElement.java
@@ -9,6 +9,11 @@
 
     protected final ElementId id;
 
+    // For serialization
+    public AbstractElement() {
+        id = null;
+    }
+
     /**
      * Creates a network element attributed to the specified provider.
      *