@Override
    public void onChange(final PermissionChangeEvent event) {
        if (event.getUserRef() != null) {
            cacheByUuid.invalidate(event.getUserRef().getUuid());
        }
    }
Gets a user/group by their stroom user UUID if they exist.
stroom-security/stroom-security-impl/src/main/java/stroom/security/impl/UserCache.java