Uncaught SyntaxError: Unexpected end of JSON input at JSON.parse (<anonymous>) at xhr.onload (settings.php:169:37)

I tried to convert a string into a JSON object using JSON.parse() which caused the above error, and here is my code I hope this helps:

function get_general() {
            let site_title = document.getElementById('site_title');
            let site_about = document.getElementById('site_about');

            let shutdown_toogle = document.getElementById('shutdown-toogle');

            let xhr = new XMLHttpRequest();
            xhr.open("GET", "ajax/settings_crud.php", true);
            xhr.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');

            xhr.onload = function() {
                general_data = JSON.parse(this.responseText);

                site_title.innerText = general_data.site_title;
                site_about.innerHTML = general_data.site_about;

                site_title_inp.value = general_data.site_title;
                site_about_inp.value = general_data.site_about;

                if (general_data.shutdown == 0) {
                    shutdown_toogle.checked = false;
                    shutdown_toogle.value = 0;
                } else {
                    shutdown_toogle.checked = true;
                    shutdown_toogle.value = 1;
                }
            }

            xhr.send('get_general');
        }

and this is the file of settings_crud.php , which is used to store the JSON structure in it so that it can be called. I hope this helps:

<?php


require('../inc/db_config.php');
require('../inc/essentials.php');
adminLogin();

if (isset($_POST['get_general'])) {
    $q = "SELECT * FROM 'settings' WHERE 'sr_no'=?";
    $values = [1];
    $res = select($q, $values, "i");
    $data = mysqli_fetch_assoc($res);
    $json_data = json_encode($data);
    echo $json_data;
}

if (isset($_POST['upd_general'])) {
    $frm_data = filteration($_POST);

    $q = "UPDATE `settings` SET `site_title` = ?, `site_about` = ? WHERE `sr_no` = ?";
    $values = [$frm_data['site_title'], $frm_data['site_about'], 1];
    $res = update($q, $values, "ssi");
    echo $res;
}


if (isset($_POST['upd_shutdown'])) {
    $frm_data = ($_POST['upd_shutdown']==0) ? 1 : 0 ;

    $q = "UPDATE `settings` SET `shutdown` = ? WHERE `sr_no` = ?";
    $values = [$frm_data, 1];
    $res = update($q, $values, "ii");
    echo $res;
}