Unable to apply sort or filter operations on table in static html mode

[azure31]

Environment

• Operating System: Ubuntu
• Python Version: 3.6.7
• How did you install Qgrid: pip
• Python packages: notebook==5.7.8, qgrid==1.1.1, ipywidgets==7.4.2

Description of Issue

• What did you expect to happen?

I'm trying to apply sort and filter operations on qgrid table in static html mode but it doesn't seem to be working. Please see the screenshot (columns C, E, F of table show loading icon)

The developer console shows that qgrid.js and custom.css files were not found.
jupyter nbextension list output shows jupyter-js-widgets/extension and qgrid/extension enabled

Reproduction Steps

1. Create a qgrid table widget.
2. Save notebook widget state and convert notebook to html