bug#39558: 26.3; icomplete and display *Completions* in child frame

[Andrii Kolomoiets]

1. emacs -Q
2. evaluate
(customize-set-variable
 'display-buffer-alist
 '(("\\*Completions\\*" display-buffer-in-child-frame)))
3. M-x icomplete-mode
4. M-x f<TAB>

Error in post-command-hook (icomplete-post-command-hook): (wrong-type-argument 
number-or-marker-p nil)

In GNU Emacs 26.3 (build 1, x86_64-apple-darwin18.2.0, NS appkit-1671.20 
Version 10.14.3 (Build 18D109))
 of 2019-09-02 built on builder10-14.porkrind.org
Windowing system distributor 'Apple', version 10.3.1894

Recent messages:
(("\\*Completions\\*" display-buffer-in-child-frame))
Icomplete mode enabled
Making completion list...
In #<buffer  *Minibuf-1*> (w=#<window 5 on *Completions*>): nil
Error in post-command-hook (icomplete-post-command-hook): (wrong-type-argument 
number-or-marker-p nil)