Cannot read property ‘getAttribute‘ of null

        function popUp(winURL){
            window.open(winURL,"popup","width=320,height=480");
        }
        
        window.onload = prepareLinks;
        function prepareLinks(){
            var a = document.getElementById("a");
            if(a.getAttribute("class") == "popup"){
                a.onclick = function(){
                    popUp(this.getAttribute("href"));
                    return false;
                }
            }
        }

    <a href="https://cn.bing.com/" class="popup" >Example</a>