Fix some failing unit tests

src/main/java/net/floodlightcontroller/devicemanager/internal/DeviceManagerImpl.java

@@ -918,8 +918,9 @@ public class DeviceManagerImpl implements
             int entityindex = -1;
             if ((entityindex = device.entityIndex(entity)) >= 0) {
                 // update timestamp on the found entity
-                device.entities[entityindex].
-                    setLastSeenTimestamp(entity.getLastSeenTimestamp());
+                Date lastSeen = entity.getLastSeenTimestamp();
+                if (lastSeen == null) lastSeen = new Date();
+                device.entities[entityindex].setLastSeenTimestamp(lastSeen);
                 break;
             } else {                
                 Device newDevice = allocateDevice(device, entity, classes);

src/main/java/net/floodlightcontroller/devicemanager/internal/Entity.java

@@ -148,7 +148,8 @@ public class Entity implements Comparable<Entity> {
      * @see {@link Entity#activeSince}
      */
     public void setLastSeenTimestamp(Date lastSeenTimestamp) {
-        if ((activeSince.getTime() +  ACTIVITY_TIMEOUT) <
+        if (activeSince == null ||
+            (activeSince.getTime() +  ACTIVITY_TIMEOUT) <
                 lastSeenTimestamp.getTime())
             this.activeSince = lastSeenTimestamp;
         this.lastSeenTimestamp = lastSeenTimestamp;