public LegacyFilterConfig addToDefaultList(final String... values) {
        if (get(null) == null) {
            put(null, new ArrayList<>());
        }
        get(null).addAll(Arrays.asList(values));
        return this;
    }
IF this config has only one single String value then this value is returned. Otherwise null is returned
main/src/main/java/cgeo/geocaching/utils/config/LegacyFilterConfig.java