ONOS-6785
ServiceNotFoundException in ONOS shutdown

Change-Id: I793a6fc34a8b7e9a3b7ff0010c9adfaa3e7527c5
diff --git a/web/gui/src/main/java/org/onosproject/ui/impl/UiWebSocket.java b/web/gui/src/main/java/org/onosproject/ui/impl/UiWebSocket.java
index cdd84ce..ded8806 100644
--- a/web/gui/src/main/java/org/onosproject/ui/impl/UiWebSocket.java
+++ b/web/gui/src/main/java/org/onosproject/ui/impl/UiWebSocket.java
@@ -214,9 +214,12 @@
 
     @Override
     public synchronized void onClose(int closeCode, String message) {
-        tokenService().revokeToken(sessionToken);
-        log.info("Session token revoked");
-
+        try {
+            tokenService().revokeToken(sessionToken);
+            log.info("Session token revoked");
+        } catch (ServiceNotFoundException e) {
+            log.error("Unable to reference UiTokenService");
+        }
         sessionToken = null;
 
         topoSession.destroy();
@@ -410,11 +413,7 @@
     }
 
     private UiTokenService tokenService() {
-        UiTokenService service = directory.get(UiTokenService.class);
-        if (service == null) {
-            log.error("Unable to reference UiTokenService");
-        }
-        return service;
+        return directory.get(UiTokenService.class);
     }
 
     // sends the collated localization bundle data up to the client.