我的react裡面有一段程式碼一直報error

但是換到react以外的地方測試就正常

怎麼改都報錯

程式碼如下 :

    let modal = document.getElementById('myModal_logicroute');
    let btn = document.getElementById("myBtn_logicroute");
    let span = document.getElementsByClassName("close")[0];
    btn.onclick = function() {
        modal.style.display = "block";
    }
    span.onclick = function() {
        modal.style.display = "none";
    }
    window.onclick = function(event) {
        if (event.target == modal) {
            modal.style.display = "none";
        }
    }
    
    

error msg :

Uncaught TypeError: Cannot set property 'onclick' of undefined

Uncaught TypeError: Cannot read property 'style' of null