private static void throwOnlyJsonSupportedException(FieldBase field, String fieldType) {
        throw new DeserializationException("Field '"+ (field != null ? field.getName() : "null") + "': "
                + "XML input for fields of type " + fieldType + " is not supported. Please use JSON input instead.");
    }
Optional test and set condition. Common for document/update/remove elements
This variable is either set in VespaXMLFieldReader#read (reader for document)
or in VespaXMLUpdateReader#read (reader for update).
document/src/main/java/com/yahoo/vespaxmlparser/VespaXMLFieldReader.java