private Field lookupField(String fieldName) {
        Field field = schema.field(fieldName);
        if (field == null)
            throw new DataException(fieldName + " is not a valid field name");
        return field;
    }
Validates that this struct has filled in all the necessary data with valid values. For required fields
without defaults, this validates that a value has been set and has matching types/schemas. If any validation
fails, throws a {@link DataException}.
connect/api/src/main/java/org/apache/kafka/connect/data/Struct.java