public interface AddonMapper extends Function<Addon, AddonProxy> {
    @Override
    @Mapping(source = "addonCategory.value", target = "addonCategory")
    @Mapping(source = "addonCategory.translation", target = "addonCategoryTranslated")
    AddonProxy apply(Addon addon);
}

// Generates private method like that, but this method should be STATIC:

private String addonAddonCategoryValue(Addon addon) {
        if ( addon == null ) {
            return null;
        }
        TranslatedValue addonCategory = addon.getAddonCategory();
        if ( addonCategory == null ) {
            return null;
        }
        String value = addonCategory.getValue();
        if ( value == null ) {
            return null;
        }
        return value;
    }