Fixed NPE in persistent map; however the map functionality still appears to be broken.

Change-Id: I599d66c21230675d63871757f115cbcac21a12e5
diff --git a/core/store/persistence/src/main/java/org/onosproject/persistence/impl/PersistentMap.java b/core/store/persistence/src/main/java/org/onosproject/persistence/impl/PersistentMap.java
index daf594d..23095f4 100644
--- a/core/store/persistence/src/main/java/org/onosproject/persistence/impl/PersistentMap.java
+++ b/core/store/persistence/src/main/java/org/onosproject/persistence/impl/PersistentMap.java
@@ -118,7 +118,8 @@
     @Override
     public V get(Object key) {
         checkNotNull(key, "Key cannot be null.");
-        return serializer.decode(items.get(serializer.encode(key)));
+        byte[] bytes = items.get(serializer.encode(key));
+        return bytes == null ? null : serializer.decode(bytes);
     }
 
     @Override