transient--describe-function: Handle describe-function overrides

`transient--describe-function' captures the help buffer via
`temp-buffer-window-setup-hook'. This works with the built-in
`describe-function', which displays its output via
`with-help-window' and therefore triggers the hook.

However, a common pattern in the Emacs community is to override
`describe-function' with a function provided by a packages like
`helpful'.  `helpful-function' creates and displays its buffer
via `pop-to-buffer' rather than `with-help-window', so
`temp-buffer-window-setup-hook' never fires and `buffer' stays
nil, causing (set-buffer nil) to signal `wrong-type-argument'.

See #431.

Author: benthamite

lisp/transient.el

@@ -5314,7 +5314,8 @@ Select the help window, and make the help buffer current and return it."
           (cons (lambda () (setq buffer (current-buffer)))
                 temp-buffer-window-setup-hook)))
     (describe-function fn)
-    (set-buffer buffer)))
+    (when buffer
+      (set-buffer buffer))))
 
 (defun transient--show-manual (manual)
   (info manual))