Unit tests for the flow objective store

Change-Id: I6223d973bba5e680396c338a3d8d0663a028d02b
diff --git a/core/api/src/test/java/org/onosproject/store/service/TestConsistentMap.java b/core/api/src/test/java/org/onosproject/store/service/TestConsistentMap.java
index 3573230..c45f696 100644
--- a/core/api/src/test/java/org/onosproject/store/service/TestConsistentMap.java
+++ b/core/api/src/test/java/org/onosproject/store/service/TestConsistentMap.java
@@ -82,7 +82,12 @@
 
     @Override
     public Versioned<V> get(K key) {
-        return version(map.get(key));
+        V value = map.get(key);
+        if (value != null) {
+            return version(value);
+        } else {
+            return null;
+        }
     }
 
     @Override