[ONOS-4930] Fix NPE from ScalableGateway caused by the event triggered before map creation

Change-Id: I4b4890274bafbac8eae9059a9ceaf390ab290a9b

@@ -122,11 +122,6 @@ public class ScalableGatewayManager implements ScalableGatewayService {
     @Activate
     protected void activate() {
         appId = coreService.registerApplication(APP_ID);
-        configRegistry.registerConfigFactory(configFactory);
-        configService.addListener(configListener);
-        deviceService.addListener(internalDeviceListener);
-
-        selectGroupHandler = new SelectGroupHandler(groupService, deviceService, driverService, appId);
 
         gatewayNodeMap = storageService.<DeviceId, GatewayNode>consistentMapBuilder()
                 .withSerializer(Serializer.using(GATEWAYNODE_SERIALIZER.build()))
@@ -134,6 +129,12 @@ public class ScalableGatewayManager implements ScalableGatewayService {
                 .withApplicationId(appId)
                 .build();
 
+        configRegistry.registerConfigFactory(configFactory);
+        configService.addListener(configListener);
+        deviceService.addListener(internalDeviceListener);
+
+        selectGroupHandler = new SelectGroupHandler(groupService, deviceService, driverService, appId);
+
         log.info("started");
     }