ONOS-4972: Augmented UiModelEvents to hold context and memo about the event subject.

Change-Id: Id0e28d8d5d3eb80fba36e0392cc80167effd39bc
diff --git a/core/api/src/main/java/org/onosproject/ui/model/topo/UiDeviceLink.java b/core/api/src/main/java/org/onosproject/ui/model/topo/UiDeviceLink.java
index 3eaa827..75e2560 100644
--- a/core/api/src/main/java/org/onosproject/ui/model/topo/UiDeviceLink.java
+++ b/core/api/src/main/java/org/onosproject/ui/model/topo/UiDeviceLink.java
@@ -61,12 +61,12 @@
 
     @Override
     public String endPortA() {
-        return portA.toString();
+        return portA == null ? null : portA.toString();
     }
 
     @Override
     public String endPortB() {
-        return portB.toString();
+        return portB == null ? null : portB.toString();
     }