getBoundingClientRect表示未定义

Question

我正在构建一个网站，一旦用户向下滚动，我想要动画。我遇到了getBoundingClientRect的问题，因为它说它在页面的控制台中是未定义的。谢谢你的帮助！

var pointsArray = document.getElementsByClassName('point');

var animatePoints = function (points) {

    for (var i = 0; i < points.length; i++) {
        points[i].style.opacity = 1;
        points[i].style.transform = "scaleX(1) translateY(0)";
        points[i].style.msTransform = "scaleX(1) translateY(0)";
        points[i].style.WebkitTransform = "scaleX(1) translateY(0)";
    }
};

window.onload = function () {

    var sellingPoints = document.getElementsByClassName('selling-points')[0];
    var scrollDistance = sellingPoints.getBoundingClientRect().top - window.innerHeight + 200;

     window.addEventListener("scroll", function(event) {

         if (document.documentElement.scrollTop || document.body.scrollTop >= scrollDistance) {
             animatePoints(pointsArray);   
         }

    });
}