bug#69501: 30.0.50; customize-mode error with substring completion style

[Eli Zaretskii <eliz@gnu.org>]

> Date: Sat, 02 Mar 2024 09:59:48 +0100
> From:  Eshel Yaron via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> 
> With both Emacs 29.2 and with the current master:
> 
> 1. emacs -Q
> 2. (setq completion-styles '(substring))
> 3. C-u M-x customize-mode TAB
> 4. "Assertion failed: (stringp suffix)"
> 
> I not sure exactly why this error occurs, but IIUC it has to do with the
> fact that customize-mode let-binds completion-regexp-list.  I suspect
> that the substring completion style doesn't handle that correctly.
> 
> In general, let-binding completion-regexp-list around completing-read is
> not a great idea IMO because it affects recursive minibuffers as well.
> So perhaps the best solution is to avoid doing that in customize-mode,
> and to rely only on the predicate argument of completing-read instead?

Stefan and Mauro, any suggestions?