> In case it helps, let me repeat what I wrote at the outset for this bug:
 > this crash came from a session started with `emacs -Q', *not* from my
 > setup (so no standalone minibuffer etc.).

IIUC at least one frame didn't have a minibuffer since otherwise this
error could not have occured.  FWIW the attached patch should fix it but
I don't have any idea how to test it.

martin