fix: deserialize state before updating model on reconnect

[maartenbreddels]

This is a backport of #3549 from 8.x to the 7.x branch.

When jupyterlab resumes from a sleeping computer, it will reconnect, and instead of creating the new comms, it will update the models. Like #3549 we did not deserialize the state in this rare condition. This caused a model widget to be passed as a string, which later on caused model.state_change.then to fail, since model.state_change is undefined. On the js console, it would show:

Error setting state: Cannot read properties of undefined (reading ‘then’)

This caused widgets not to appear.

Privately reported, and also likely the cause of spacetelescope/jdaviz#1977.

[jasongrout]

Thanks!

The docs build failure is unrelated.