Fix: resolve a set of NPEs due to security group activation

Change-Id: Idfd93c80f710e85370f9054d93b9b7e96c77fd49
diff --git a/apps/openstacknetworking/app/src/main/java/org/onosproject/openstacknetworking/impl/OpenstackSwitchingArpHandler.java b/apps/openstacknetworking/app/src/main/java/org/onosproject/openstacknetworking/impl/OpenstackSwitchingArpHandler.java
index 59241cb..dceb7ca 100644
--- a/apps/openstacknetworking/app/src/main/java/org/onosproject/openstacknetworking/impl/OpenstackSwitchingArpHandler.java
+++ b/apps/openstacknetworking/app/src/main/java/org/onosproject/openstacknetworking/impl/OpenstackSwitchingArpHandler.java
@@ -651,6 +651,11 @@
         }
 
         private void setDefaultArpRule(OpenstackNode osNode, boolean install) {
+
+            if (getArpMode() == null) {
+                return;
+            }
+
             switch (getArpMode()) {
                 case ARP_PROXY_MODE:
                     setDefaultArpRuleForProxyMode(osNode, install);