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

[martin rudalics]

> 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.

Please report this to the author(s) of that package.

> 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.
>
> The fix I'm using is:
>
>      (if (and (display-multi-font-p (frame-parameter frame 'display))
>               (stringp default-font)  ; <<<---!!!

What is `default-font'?

>         (not (string-equal (frame-parameter frame 'font) default-font)))
>
> But there could be other considerations to performing things in another way.

martin