public String toDisplayString() {
        if (displayName != null) {
            return displayName;
        } else if (subjectId != null) {
            return subjectId;
        } else {
            return NullSafe.requireNonNullElseGet(fullName, () ->
                    "{" + uuid + "}");
        }
    }
@return "Group" or "User" depending on type.
stroom-util-shared/src/main/java/stroom/util/shared/UserRef.java