bug#36588: Unable to revert M-x apropos help buffer

[Stefan Kangas]

I get an error trying to revert the apropos help buffer on Emacs 27
(current master).  I could also reproduce this on 26.1.

Steps to reproduce:
0. emacs -Q
1. M-x apropos RET foo RET
2. C-x o
3. g yes RET
4. Error:
apply: Symbol’s function definition is void: nil

Backtrace:
Debugger entered--Lisp error: (void-function nil)
  nil()
  apply(nil nil)
  help-mode-revert-buffer(t nil)
  revert-buffer(t)
  funcall-interactively(revert-buffer t)
  #<subr call-interactively>(revert-buffer nil nil)
  apply(#<subr call-interactively> revert-buffer (nil nil))
  call-interactively@ido-cr+-record-current-command(#<subr
call-interactively> revert-buffer nil nil)
  apply(call-interactively@ido-cr+-record-current-command #<subr
call-interactively> (revert-buffer nil nil))
  call-interactively(revert-buffer nil nil)
  command-execute(revert-buffer)

GNU Emacs 27.0.50 (build 5, x86_64-apple-darwin15.6.0, NS
appkit-1404.47 Version 10.11.6 (Build 15G22010)) of 2019-07-11

Thanks,
Stefan Kangas