Hi and welcome to Playcanvas forums!
Hmm, the rigidbody-extensions.js was an old name of the extension file. I no longer can edit the original post, unfortunately. The filename of the extension is convex-cast.js and should be used together with the Ammo build, found in the example project.
I forked your project, copied Ammo folder and convex-cast.js from my project:
https://playcanvas.com/editor/scene/1054547
I cannot reproduce the error from your project. Please, check.