Widgets tabs are blank when opening a second time

[abauske]

I am using a jupyterhub/singleuser instance with nbgrader installed.
When you open any nbgrader tab/widget a second time (so open once, close using the x, and open again) then the Tab is blank:

Mind the js error in browser console.

I tried to trace the error in source code by building the dev version of js, so I could fix it and prepare a PR. Unfortunately I wasn't able to get the assignment_list widget to work (I followed this documentation). The Nbgrader entry in the topmost row just won't show (maybe you can also nudge me in the correct direction here, so I might be able to prepare a PR)

Operating system

Windows, Docker

nbgrader --version

0.8.1

jupyterhub --version (if used with JupyterHub)

3.1.0

jupyter notebook --version

6.5.2

Expected behavior

You should be able to open and close any tab as often as you want without it going blank.

Actual behavior

After opening the tab the second time it is blank and can't be closed or modified in any way. When reloading the window all other tabs are persisted but this tab is just closed.

Steps to reproduce the behavior

Open e.g. Assignment List from Nbgrader entry in topmost row -> the Assignment List tab opens. Close this tab. Try opening it again -> blank tab