Missing default directory on install-shutdown save


When installing a package that is already loaded, RStudio recommends that it save the session and do a shutdown/restart. I usually accept this recommendation. (the reason I do such installs is usually that I have added one or more packages to a list of packages I install in a new project). I find that I often get some message about not being able to save part of my session. As an example, here is the most recent one:

"Error saving session (plots_dir): The system cannot find the path specified"

I did not create and do not consciously make any use of a directory of that name, so I assume it is a default directory of some sort, and I wonder what is causing the error and how to prevent it.