Javascript - 未捕获TypeError：无法读取null

Question

我正在尝试在我的网站上执行javascript，但我得到了：

  

未捕获的TypeError：无法读取null的“getContext”属性

我不知道我做错了什么。它只在一个网站上完美执行..

这是脚本代码：

var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
canvas.width = window.innerWidth;
canvas.height = window.innerHeight;
var canvasWidth = canvas.width;
var canvasHeight = canvas.height;
var dots = [];
var r = Math.floor(Math.random() * 255) + 125;
var g = Math.floor(Math.random() * 255) + 125;
var b = Math.floor(Math.random() * 255) + 125;
var radius = Math.floor(Math.random() * 2) + 1;

function Dot(xCoordinate, yCoordinate, radiusValue, rValue, gValue, bValue, aValue) {
    this.x = xCoordinate;
    this.y = yCoordinate;
    this.radius = radiusValue;
    this.r = rValue;
    this.g = gValue;
    this.b = bValue;
    this.a = aValue;
}

function dotFilter(element) {
    return element.y >= 0 && element.a >= 0;
}

function drawDots() {
    ctx.fillStyle = "rgb(255,255,255)";
    ctx.fillRect(0, 0, canvasWidth, canvasHeight);
    if (dots.length === 0) {
        return;
    }
    dots = dots.filter(dotFilter);
    for (i = 0; i < dots.length; i = i + 1) {
        dots[i].y = dots[i].y - 12;
        dots[i].a = dots[i].a - .03;
        ctx.save();
        ctx.beginPath();
        ctx.arc(dots[i].x, dots[i].y, dots[i].radius, 0, 4 * Math.PI, true);
        ctx.fillStyle = "rgba(" + dots[i].r + "," + dots[i].g + "," + dots[i].b + "," + dots[i].a + ")";
        ctx.fill();
        ctx.restore();
    }
}
canvas.onmousemove = function(e) {
    var mouseX, mouseY;
    if (e.offsetX) {
        mouseX = e.offsetX;
        mouseY = e.offsetY;
    } else if (e.layerX) {
        mouseX = e.layerX;
        mouseY = e.layerY;
    }
    r = Math.floor(Math.random() * 255) + 50;
    g = Math.floor(Math.random() * 255) + 50;
    b = Math.floor(Math.random() * 255) + 50;
    radius = Math.floor(Math.random() * 2) + 1;
    dots.push(new Dot(mouseX - 12, mouseY - 12, radius, r, g, b, 1));
};

function clearScreen() {
    ctx.fillStyle = "rgb(255,255,255)";
    ctx.fillRect(0, 0, canvasWidth, canvasHeight);
}
ctx.fillStyle = "rgb(255, 255, 255)";
ctx.fillRect(0, 0, canvasWidth, canvasHeight);
window.setInterval(drawDots, 70);