revoke subscription not working for websocket

Hi,

I am using websocket (ringcentral-websocket-0.2.0.jar) and invoking revoke method in subscription to unsubscribe but it throws null pointer exception at line subscription.revoke() . Can you suggest me how to fix this issue or how to unsubscribe current websocket subscription when customer deauthorizes access

our code:

Subscription subscription = new Subscription(setUpRestClient(account), eventFilters, this::eventConsumer);
subscription.revoke();