Ah, sorry, I thought I had covered that.
Resetting the window layout results in SlickEdit exiting without comment.
A second attempt results in a pop-up box "Are you sure you want to restore the default window layout?" and selecting yes on that results in SlickEdit exiting without comment.
When I open SlickEdit the list of windows only contains ft.c which is a file that exists and which I can edit, and have edited since this issue arose, with vim or emacs.