bug#51199: describe-key on a toolbar item throws an error

[Po Lu]

In master and the latest emacs-28, I get:
mouse-minibuffer-check: Wrong type argument: window-valid-p, #<frame 
*Backtrace* - GNU Emacs at shredder 0x11509a60f7e8>
When I press C-h k and then click a button on the toolbar.  (This won't
be reproducible if you have toolkit tool bars).

This used to work in Emacs 27, and is very useful when I want to check
what a button on the toolbar does.  It would be appreciated if someone
looked into this.  Thanks.