ONOS-4971: Synthetic Link Data -- WIP

- Enhancing UiRegion to capture the hierarchical (parent/child) relationships captured in the UiTopoLayouts.

Change-Id: I152e0d52d4580b14b679f3387402077f16f61e6a
diff --git a/core/api/src/main/java/org/onosproject/ui/model/topo/UiTopology.java b/core/api/src/main/java/org/onosproject/ui/model/topo/UiTopology.java
index 585e658..708fd18 100644
--- a/core/api/src/main/java/org/onosproject/ui/model/topo/UiTopology.java
+++ b/core/api/src/main/java/org/onosproject/ui/model/topo/UiTopology.java
@@ -151,7 +151,8 @@
 
 
     /**
-     * Returns all regions in the model.
+     * Returns all regions in the model (except the
+     * {@link #nullRegion() null region}).
      *
      * @return all regions
      */
@@ -177,7 +178,7 @@
      * @return corresponding UI region
      */
     public UiRegion findRegion(RegionId id) {
-        return regionLookup.get(id);
+        return UiRegion.NULL_ID.equals(id) ? nullRegion() : regionLookup.get(id);
     }
 
     /**