GLTFExporter is undefined

Hello,
I tried to export the scene as GLTF as described in the documentation (https://www.soft8soft.com/docs/examples/exporters/GLTFExporter.html), but I had the following issues:

if I call

var exporter = new v3d.GLTFExporter()

I get this error:
TypeError: v3d.GLTFExporter is not a constructor

I tried to load the exporter like this (inside runCode function):

var script=document.createElement('script');
    script.src='https://threejs.org/examples/js/exporters/GLTFExporter.js';
    script.onload = function () {
      console.log("exporter loaded");
      app.exporter = new THREE.GLTFExporter({onlyVisible:true});
    }
    document.head.appendChild(script);

this step was ok, but when I export (inside an ExternalInterface function) I get this other error:

Uncaught TypeError: Cannot convert undefined or null to object
    at Function.keys (<anonymous>)
    at processMesh (GLTFExporter.js:1204)
    at processNode (GLTFExporter.js:1584)
    at processNode (GLTFExporter.js:1614)
    at processScene (GLTFExporter.js:1679)
    at processObjects (GLTFExporter.js:1716)
    at processInput (GLTFExporter.js:1742)
    at THREE.GLTFExporter.parse (GLTFExporter.js:1760)