VERTEX_SHADER & FRAGMENT_SHADER возвращаются как ошибка типа.Как исправить?

Question

Я слежу за книгой по созданию игрового движка WebGL. Я создаю шейдеры, но каждый раз, когда я пытаюсь загрузить / скомпилировать их, я получаю следующую ошибку:

SimpleShader.js: 28 Uncaught TypeError: Невозможно прочитать свойство 'VERTEX_SHADER' с нулевым значением

Я также получаю ту же ошибку с фрагментным шейдером Вот простой код шейдера

/* 
 * File: SimpleShader.js
 * 
 * Implements a SimpleShader object.
 * 
 */
/*jslint node: true, vars: true */
/*global gEngine: false, document: false, alert: false */

"use strict";  // Operate in Strict mode such that variables must be declared before used!

function SimpleShader(fragmentShaderPath, vertexShaderPath) {
    // instance variables
    // Convention: all instance variables: mVariables
    this.mCompiledShader = null;  // reference to the compiled shader in webgl context  
    this.mShaderVertexPositionAttribute = null; // reference to SquareVertexPosition within the shader
    this.mPixelColor = null;                    // reference to the pixelColor uniform in the fragment shader

    var gl = gEngine.Core.getGL();

    // start of constructor
    //
    // Load and compile vertex and fragment shaders
    var fragmentShader = this._loadAndCompileShader(fragmentShaderPath, gl.FRAGMENT_SHADER);
    var vertexShader = this._loadAndCompileShader(vertexShaderPath, gl.VERTEX_SHADER);
    
    // Creat eand link the shaders into a program
    this.mCompiledShader = gl.createProgram();
    gl.attachShader(this.mCompiledShader, vertexShader);
    gl.attachShader(this.mCompiledShader, fragmentShader);
    gl.linkProgram(this.mCompiledShader);
    
    // check for errors
    if(!gl.getProgramParameter(this.mCompiledShader, gl.LINK_STATUS)) {
        alert("Error Linking Shader");
        return null;
    }
    
    // gets a reference to the aSquareVertexPosition attribute
    this.mShaderVertexPositionAttirbute = gl.getAttribLocation(this.mCompiledShader, "aSquareVertexPosition");
    // Activates the vertex buffer loaded in Engine.Core_VertexBuffer
    gl.bindBuffer(gl.ARRAY_BUFFER, gEngine.VertexBuffer.getGLVertexRef());
   
    // Describe the characteristics of the vertex position attribute
    gl.vertexAttribPointer(this.mShaderVertexPositionAttribute, 
    3, // each element is a 3-float
    gl.FLOAT, // data type is FLOAT
    false, // if the content is normailed vectors
    0, // numbers of bytes to skip in between elements
    0); // offsets to the first element
    // gets reference to the uniform 
    this.mPixelColor = gl.getUniformLocation(this.mCompiledShader, "uPixelColor");
    
    
    
    
}

SimpleShader.prototype.getShader = function() { return this.mCompiledShader; };

// Activate the shader for rendering
SimpleShader.prototype.activateShader = function (pixelColor) {
    var gl = gEngine.Core.getGL();
    gl.useProgram(this.mCompiledShader);
    gl.bindBuffer(gl.ARRAY_BUFFER, gEngine.VertexBuffer.getGLVertexRef());
    gl.vertexAttribPointer(this.mShaderVertexPositionAttribute,
        3,              // each element is a 3-float (x,y.z)
        gl.FLOAT,       // data type is FLOAT
        false,          // if the content is normalized vectors
        0,              // number of bytes to skip in between elements
        0);             // offsets to the first element
    gl.enableVertexAttribArray(this.mShaderVertexPositionAttribute);
    gl.uniform4fv(this.mPixelColor, pixelColor); // Copies the floating point values
    
};

// Returns a compiled Shader from a shader in the dom
// the ID is the ID of the script
SimpleShader.prototype._loadAndCompileShader = function(filePath, shaderType) {
    var gl = gEngine.Core.getGL();
    var xmlReq, shaderSource = null, compiledShader = null;
    
    // Request the text from the given location
    xmlReq = new XMLHttpRequest();
    xmlReq.open('GET', filePath, false);
    try {
        xmlReq.send();
    } catch(error) {
        alert("Failed to load shader: " + filePath);
        return null;
    }
    shaderSource = xmlReq.responseText;
    
    if(shaderSource === null) {
        alert("WARNING: Loading of: " + filePath + "Failed");
        return null;
    }
    
    // create the shader based on the shader type: vertex / Fragment
    compiledShader = gl.createShader(shaderType);
    
    //compile the created shader
    gl.shaderSource(compiledShader, shaderSource);
    gl.compileShader(compiledShader);
    
    // check for errors
    if(!gl.getShaderParameter(compiledShader, gl.COMPILE_STATUS)) {
        var error = "ERROR: Shader Compiling Error: " + gl.getShaderInfoLog(compiledShader);
        alert(error);
        console.log(error);
    }
    return compiledShader;
};