Can't install and use properly pythreejs - installation

Question

I installed pythreejs following the instructions on the official website
pip3 install pythreejs
jupyter nbextension install --user --py pythreejs
jupyter nbextension enable --user --py pythreejs
Looking at the output of jupyter nbextension list, everything looks fine:
Known nbextensions:
config dir: /home/gael/.jupyter/nbconfig
notebook section
jupyter-threejs/extension enabled
- Validating: OK
But when I start the server (jupyter notebook), create a new (Python3) notebook, and try to run the example code
from pythreejs import *
import numpy as np
from IPython.display import display
from ipywidgets import HTML, Text, Output, VBox
from traitlets import link, dlink
ball = Mesh(geometry=SphereGeometry(radius=1, widthSegments=32, heightSegments=24),
material=MeshLambertMaterial(color='red'),
position=[2, 1, 0])
c = PerspectiveCamera(position=[0, 5, 5], up=[0, 1, 0],
children=[DirectionalLight(color='white', position=[3, 5, 1], intensity=0.5)])
scene = Scene(children=[ball, c, AmbientLight(color='#777777')])
renderer = Renderer(camera=c,
scene=scene,
controls=[OrbitControls(controlling=c)])
display(renderer)
from pythreejs' official repository, it does not show me a visualization window embedded in the page. Instead, below the cell it displays a hash:
UmVuZGVyZXIoY2FtZXJhPVBlcnNwZWN0aXZlQ2FtZXJhKGNoaWxkcmVuPShEaXJlY3Rpb25hbExpZ2h0KGNvbG9yPSd3aGl0ZScsIGludGVuc2l0eT0wLjUsIHBvc2l0aW9uPSgzLjAsIDUuMCzigKY=
I also tried with jupyter lab, and with Python2, and it had exactly the same behavior.
What am I missing?
System: Ubuntu 18.04
Jupyter Notebook version: 5.7.8

Answer 1

Looking more closely at jupyter's output in the console, I read:
[W 18:55:45.544 NotebookApp] 404 GET /nbextensions/widgets/notebook/js/extension.js?v=20200107185531 (127.0.0.1) 14.69ms referer=http://localhost:8889/notebooks/Untitled.ipynb?kernel_name=python3
Following advice for this error I do:
jupyter nbextension enable --py widgtesextension
then start the server: and pythreejs works!!