how use Cesium3DTileset

1. A concise explanation of the problem you're experiencing.

I create MVC Demo, add build Folder of Cesium-1.50 to script Folder and want to create View such as below,but when I Run it got this Error:
Cesium.Cesium3DTileset is not a constructor
    at window.onload

2. A minimal code example. If you've found a bug, this helps us reproduce and repair it.

       window.onload = function () {
            var viewer = createViewer("viewer", "@Url.Content("~/Scripts")");
            var scene = viewer.scene;
            Cesium.Cartesian3.fromDegrees(53.9868, 26.5325, 0.0);

            var tileset =
                scene.primitives.add(new Cesium.Cesium3DTileset({
                    url: ‘
            scene.terrainProvider = new Cesium.CesiumTerrainProvider({
                url: ‘

3. Context. Why do you need to do this? We might know a better way to accomplish your goal.

4. The Cesium version you're using, your operating system and browser.
Cesium-1.50,Google Chrome latest version

How are you loading in Cesium? What does the “createViewer” function look like? Make sure you’re correctly including everything as described here: