“帮助”无法读取null的属性“ addEventListener”

Question

我在控制台上收到错误消息

var modal = document.getElementById('simpleModal');
var modalBtn = document.getElementById('modalBtn');
var closeBtn = document.getElementById('closeBtn');

modalBtn.addEventListener('click', openModal);

function openModal() {
  console.log(123);
}

<button id="modalBtn" class="button">Click Here</button>

<div id="simpleModal" class="modal">
  <div class="modal-content">
    <span class="closeBtn">&times;</span>
    <p> hello I am a modal>
  </div>
</div>

我正在做的是尝试使模式打开，但是我一直失败。