bug#47766: 28.0.50; choose-completion fails

[Gregory Heytings]

> In GNU Emacs 28.0.50 (build 2, x86_64-pc-linux-gnu, Motif Version 2.3.8, 
> cairo version 1.16.0) of 2021-04-14 built on maher based on master 
> 844b8949a71f180d395a237c768b22d91cf91ded (with the motif revert 
> reverted)
>
> 1. emacs -Q -eval '(setq pop-up-windows nil pop-up-frames t)'
> 2. C-x b TAB
>
> this would pop up a new *Completions* buffer in a new frame with are two 
> choices *Messages* and *scratch*
>
> 3. choose a completion (click or hit enter on one of the choices)
>
> user-error: Cannot switch buffers in minibuffer window

This is because of commit 7c2ebf6e23.