SendGrid send type text/plain and text/html

I have a service that sends email and everything works fine there, but I need to send 2 types of emails text/html and text/plain. I add this to my code:

Content plainContent = new Content("text/plain", "This is plain content");

mail.addContent(htmlContent);

mail.addContent(plainContent);

and the letters stopped coming

@Override
public void send(Message message) throws IOException {
    Personalization personalization = new Personalization();
    message.getSendTo().forEach(address -> personalization.addTo(new Email(address.getEmail(), address.getName())));

    Content htmlContent = new Content(message.getContentType(), message.getContent());

    Mail mail = new Mail();
    mail.setFrom(new Email(message.getSendFrom().getEmail(), message.getSendFrom().getName()));
    mail.addPersonalization(personalization);
    mail.setSubject(message.getSubject());
    mail.addContent(htmlContent);


    Request request = new Request();
    request.setMethod(Method.POST);
    request.setEndpoint("mail/send");
    request.setBody(mail.build());
    Response response = sendgrid.api(request);

    if (response.getStatusCode() != 202) {
        throw new RuntimeException(response.getBody());
    }
}

I found the answer to my question:

{"errors":[{"message":"If present, text/plain must be first, followed by text/html, followed by any other content.","field":"content" ,"help":null}]}

It is necessary to simply swap setContent.

First mail.addContent(plainContent);

and then mail.addContent(htmlContent);