I've pared down the code to load.  Just load the attached and then follow the
recipe:

M-x f TAB ; to display *Completions* frame.
C-]       ; to return to top level.

M-x f TAB o

Or just hit TAB twice in a row: M-x f TAB TAB.  IOW, try to type more
input in minibuffer.

This then raises the error
"Buffer is read-only #<buffer *Completions*>"