bug#48408: BUGFIX: window-default-font-height: check for nil string

[Eli Zaretskii]

> Date: Thu, 13 May 2021 21:15:19 -0400
> From: Boruch Baum <boruch_baum@gmx.com>
> 
> In package windows.el, function `window-default-font-height' calls
> function `face-font', which can return a nil value, but performs a
> `string-equal' operation on the output without checking for that
> possibiliy.
> 
> I came across this in an unusual case, of having an 'emacs -nw' server
> an 'emacsclient -nw', and opening a GUI emacsclient to connect to it.
> From that point forward until the fix, every *second* attempt to use the
> minibuffer on the 'emacsclient -nw' would fail.

This means the problem is either in window-normalize-window or in
frame-parameter or in display-graphic-p, and should be fixed there.
Can you tell what does the below produce on the TTY frame where you
have this problem?

  M-: (frame-parameter nil 'display) RET
  M-: (window-frame (window-normalize-window nil t)) RET

> The fix I'm using is:
> 
>     (if (and (display-multi-font-p (frame-parameter frame 'display))
>              (stringp default-font)  ; <<<---!!!
>            (not (string-equal (frame-parameter frame 'font) default-font)))

No, that's not the right fix, IMO.  The code already takes care of not
assuming default-font is a string when on TTY frames, but the
condition doesn't work in this case for some reason.  We need to
understand why and fix that there.

Thanks.