The most voted answer doesn't seem to work when using Jupyter Lab. This one does, however. Just copy the url into a new tab, replace 'lab' with 'tree' and hit enter to load the page. It will generate a new csrf token for your session and you're good to go!
I would suggest enabling Settings > Autosave Documents by default to avoid worrying about losing work in future. It saves very regularly so everything should be up to date before any timeouts happen anyway.
I did not need to open a new notebook. Instead, I reopened the tree, and reconnected the kernel. At some point I also restarted the kernel. – user650654 Oct 9 '19 at 0:17