public static void ensureNotInitialized(String fieldDescription, Object fieldOwner, Object fieldValue) {
        if (fieldValue != null) {
            throw new IllegalStateException(fieldDescription + " of " + fieldOwner +
                                            " cannot be changed, it is already set " + "to " + fieldValue);
        }
    }
Throws an IllegalStateException if the given field value
is initialized (not null)
vespajlib/src/main/java/com/yahoo/protect/Validator.java