(minor) Out-of-band update in notebooks causes uncaught exception in browser console

[dlqqq]

#93 introduces out-of-band update handling in notebooks. However, sometimes, when an out-of-band update occurs, this exception is logged in the browser console:

This likely has something to do with the notebook virtualization logic in JupyterLab, as the exception is being emitted from the WindowedList class in JupyterLab.

Also, this likely relates to / causes #97.