Cannot read property 'material' of undefined Three.js
<p>I can not understand why it shows me the following error:</p>

<blockquote>
  <p>Can not read property ' material ' of undefined " at line "
      scene.getObjectByName ( " cube " ) . material.opacity = control.opacity ; "</p>
</blockquote>

<p>My code:</p>

<pre><code>var scene = new THREE.Scene();
var camera = new THREE.PerspectiveCamera( 75, window.innerWidth/window.innerHeight, 0.1, 1000 );
var renderer = new THREE.WebGLRenderer({ antialias: true });
renderer.setSize( window.innerWidth, window.innerHeight );
renderer.shadowMap.enabled = true;
document.body.appendChild( renderer.domElement );

var axisHelper = new THREE.AxisHelper( 40); 
scene.add( axisHelper );


var cubeGeometry = new THREE.CubeGeometry( 5, 5, 5 );
var cubeMaterial = new THREE.MeshLambertMaterial( { color: 0x00ff00 } );
var cube = new THREE.Mesh( cubeGeometry, cubeMaterial );

cube.position.x = 0;
cube.position.y = 5;
cube.position.z = 0;
cube.castShadow= true;
scene.add( cube );   

var planeGeometry = new THREE.PlaneGeometry( 50, 50, 50  )
var planeMaterial = new THREE.MeshLambertMaterial( { color: 0Xcccccc } );
var plane = new THREE.Mesh(planeGeometry,planeMaterial);
plane.receiveShadow = true;
plane.rotation.x=-0.5*Math.PI;
plane.position.x=0 
plane.position.y=0
plane.position.z=0
scene.add( plane )

var ambientLight = new THREE.AmbientLight(0x0c0c0c);
scene.add(ambientLight);

var spotLight = new THREE.SpotLight( 0xffffff );
var lightHelper = new THREE.SpotLightHelper( spotLight );
spotLight.position.set( 10,50,20);
spotLight.castShadow = true;
spotLight.shadow.mapSize.width = 1024;
spotLight.shadow.mapSize.height = 1024;

//scene.add (lightHelper)
scene.add( spotLight );


camera.position.x = 20;
camera.position.y = 15;
camera.position.z = 20;
camera.lookAt(scene.position)


function addControlGui(controlObject){
    var gui = new dat.GUI();
    gui.add(controlObject,"rotationSpeed",-0.01, 0.01);
    gui.add(controlObject,"opacity",0.1,1);
    gui.addColor(controlObject,"color");
}

var control = new function(){
    this.rotationSpeed =0.005 ;
    this.opacity= 0.6;
    this.color = cubeMaterial.color.getHex();
}

var render = function () {         scene.getObjectByName("cube").material.opacity=control.opacity;
    scene.getObjectByName("cube").material.color =new       THREE.Color(control.color);
    cube.rotation.x += 0.1;
    cube.rotation.y += 0.1;
    requestAnimationFrame( render );
    renderer.render(scene, camera);
};
addControlGui(control); 
render();
</code></pre>