@Override
    public void onChange(final PermissionChangeEvent event) {
        NullSafe.consume(
                event.getUserRef(),
                UserRef::getUuid,
                cacheByUuid::invalidate);
    }
Gets a user/group by their stroom user UUID if they exist.
stroom-security/stroom-security-impl/src/main/java/stroom/security/impl/UserInfoCache.java