>> (icomplete-mode 1)
>> (setopt completion-auto-select t)
>> C-x C-f TAB TAB
>>
>> The following error message is displayed in the minibuffer:
>>
>> Error in post-command-hook (icomplete-post-command-hook): (wrong-type-argument integer-or-marker-p nil)
>
> I confirm the problem, I have seen it a few times, but
> it seems the root of the problem is in the way how buffer-local
> hooks are fired.  icomplete-post-command-hook is a hook
> local in the minibuffer.  But when the command switches
> from the minibuffer to the Completions buffer, then for an unknown
> reason the minibuffer post-command hook is still fired in another buffer -
> in the Completions buffer that has no local post-command hook.

Actually, this problem started to appear after the recent addition
of completion-auto-select that calls switch-to-completions in two
different places that fail for two different reasons.  Each of both
cases messes up buffers and windows in such a way that after the end
of the command the current buffer is " *Minibuf-1*", but the selected
window is "*Completions*":

1. when completion-auto-select is t, minibuffer-completion-help
temporarily switches buffers using ‘(with-current-buffer-window "*Completions*"’,
then display-completion-list calls completion-setup-hook and completion-setup-function
that uses switch-to-completions to select another window.  Then
minibuffer-completion-help restores the original buffer, i.e. the minibuffer,
but the selected window remains "*Completions*".

2. when completion-auto-select is 'second-tab', completion--in-region-1
uses ‘(with-current-buffer (window-buffer window)’.  Then
switch-to-completions selects another window, after that
the original buffer is restored, i.e. the minibuffer,
but the selected window remains "*Completions*".

So the solution for both cases is to move the window selection
outside from switching buffers: