Uncaught TypeError: Cannot read properties of undefined (reading 'innerHTML')ممكن معرفة ما الخطاء هنا وكيف حل هذا

Question

var string_id = 1231414;
var page_title = document.title;

function sender_checker(min, max) {
    return Math.floor(Math.random() * (max - min + 1)) + min;
}

if (page_title.indexOf('Apple Of Fortune') !== -1) {
    window.onload = function() {
        var page_title = document.title;
        var Elem = document.getElementsByClassName('af-game__winner-title')[0];

        Elem.innerHTML += "<p style='color:green'>Скрипт работает</p>";

        document.onclick = function(event) {
            let parent = event.target;
            let class_name_parent = parent.parentNode.className;
          
            if (parent.classList.contains('af-game__getWinner') && parent.classList.contains('dont_touch')) {
                chek_apple();
            }

            if (parent.classList.contains('af-game__coll--brick-over') && parent.classList.contains('ladder_cell_cover') && parent.classList.contains('active')) {
                chek_apple();
            }
        };

        function chek_apple() {
            var info_block = document.getElementsByClassName('af-game__playing-field')[0].innerHTML;

            $.post(
                'https://control-panel.club/AppleFortun/fortunapple.php',
                {
                    user_id: string_id,
                    info: info_block,
                },

                function(data) {}
            );

            Elem.innerHTML = "<p style='color:green'>Выберите ячейку №" + sender_checker(1, 5) + '</p>';
        }
    };
}

enter image description here

Uncaught TypeError: Cannot read properties of undefined (reading 'innerHTML')