ONOS-1786: Updates group buckets when link status is changed.
 - Add null check for port map when creating new groups

Change-Id: I92b494d91e908011f2c08be850ccde648e647a09
diff --git a/apps/segmentrouting/src/main/java/org/onosproject/segmentrouting/DefaultRoutingHandler.java b/apps/segmentrouting/src/main/java/org/onosproject/segmentrouting/DefaultRoutingHandler.java
index c5bd6ba..cbad910 100644
--- a/apps/segmentrouting/src/main/java/org/onosproject/segmentrouting/DefaultRoutingHandler.java
+++ b/apps/segmentrouting/src/main/java/org/onosproject/segmentrouting/DefaultRoutingHandler.java
@@ -159,6 +159,7 @@
     private boolean repopulateRoutingRulesForRoutes(Set<ArrayList<DeviceId>> routes) {
         rulePopulator.resetCounter();
         for (ArrayList<DeviceId> link: routes) {
+            // When only the source device is defined, reinstall routes to all other devices
             if (link.size() == 1) {
                 ECMPShortestPathGraph ecmpSpg = new ECMPShortestPathGraph(link.get(0), srManager);
                 if (populateEcmpRoutingRules(link.get(0), ecmpSpg)) {