py3dmol can't be converted to HTML by nbconvert [BUG]

[platinhom]

Describe the bug Download the example.ipynb, and it can be opened by Jupyter Notebook normally. However, downloading it as HTML will cause

nbconvert failed: KeyError: 'application/vnd.jupyter.widget-state+json'

To Reproduce

Steps to reproduce the behavior: Just as the describtion above.

wget https://raw.githubusercontent.com/3dmol/3Dmol.js/master/packages/py3Dmol/examples.ipynb
conda create -n 3dmoltest python=3.7
conda activate 3dmoltest
conda install -y jupyter
conda install -y -c conda-forge py3dmol
jupyter labextension install jupyterlab_3dmol
jupyter-notebook

Screenshots If applicable, add screenshots to help explain your problem.

Desktop (please complete the following information):

• OS: Ubuntu 20.04
• Browser: Chrome
• Version [e.g. 22]
  • py3dmol 1.8.1
  • nbconvert 6.4.4
  • jupyter_client 7.3.5