Why is there an error when submitting the form? - wordpress

Question

Please tell me how to solve the problem.
The site is on Wordpress, the page has several modals with forms.
They all work well. I need to configure adding an email to the Sendpulse address book, which the user enters into the field.
Installed the sendpulse package via composer: https://github.com/sendpulse/sendpulse-rest-api-php
The situation is this:
In the directory with the theme, there is a functions.php file where there is a form handler, in which you can configure sending letters with requests from forms.
In this file, I added, according to the instructions from github sendpulse, the code for sending an email to the address book:
require '../private/vendor/autoload.php';
use Sendpulse\RestApi\ApiClient;
use Sendpulse\RestApi\Storage\FileStorage;
define('API_USER_ID', 'b7****************************************');
define('API_SECRET', 'e1*************************************');
$SPApiClient = new ApiClient(API_USER_ID, API_SECRET, new FileStorage());
$bookID = '15********';
$emailForSP = ['test6#test.ru'];
$SPApiClient->addEmails($bookID, $emailForSP);
If you just add this piece to the beginning of the file, then it works out and the address is added to the book. But sending letters to mail does not work, an error is thrown: https://hsto.org/webt/v8/qn/le/v8qnled6ibyy-ltp9afwyrjv-2q.jpeg
In the error_log file the line is: https://hsto.org/webt/mk/vo/g0/mkvog03mb7olfji0qrjcsyxktf0.jpeg
Error text from main.js file: https://hsto.org/webt/cd/-n/ry/cd-nryfx06_mvgtglu-t9msp8ga.jpeg
That is, this piece of code somehow affects the processing of the form handler function. In general, I need to trigger sending an email to the book after sending a letter to the mail, but in this case nothing works at all.

Answer 1

I didn't look for the cause of the conflict for a long time. Allocated sending e-mail in the sendpulse into a separate file and added ajax post request to this file. I don’t know why I hadn’t thought of this before.
let emailForSendPulse = $form.find('[name="email"]').val();
$.post('/wp-admin/smth.php', {email: emailForSendPulse}, function(data){
console.log('Form sended');
});