[ONOS-5904] Fixed throwing an exception after creating it.

Change-Id: I4845d2cb953826a3b6e4a345bfd4d2228b941664
diff --git a/apps/vtn/vtnweb/src/main/java/org/onosproject/vtnweb/resources/RouterWebResource.java b/apps/vtn/vtnweb/src/main/java/org/onosproject/vtnweb/resources/RouterWebResource.java
index 332bd41..e78bf56 100644
--- a/apps/vtn/vtnweb/src/main/java/org/onosproject/vtnweb/resources/RouterWebResource.java
+++ b/apps/vtn/vtnweb/src/main/java/org/onosproject/vtnweb/resources/RouterWebResource.java
@@ -299,7 +299,7 @@
         checkNotNull(routerNode, JSON_NOT_NULL);
         Map<RouterId, Router> subMap = new HashMap<RouterId, Router>();
         if (!routerNode.hasNonNull("id")) {
-            new IllegalArgumentException("id should not be null");
+            throw new IllegalArgumentException("id should not be null");
         } else if (routerNode.get("id").asText().isEmpty()) {
             throw new IllegalArgumentException("id should not be empty");
         }