Strange draft problem

When I'm in the draft buffer and it is the only window and I hit \C-c \C-c,
the message is sent but the buffer is not removed.

Emacs displays the complaint: "Attempt to delete minibuffer or sole ordinary
window".  The *Messages* buffer says

delete-window: Attempt to delete minibuffer or sole ordinary window

The same thing happens with wl-draft-kill, except the draft is not killed
after the error.

I can avoid the problem with wl-draft-send-and-exit if I delete the optional
KILL-WHEN-DONE argument after wl-draft-send:

(defun wl-draft-send-and-exit ()
  "Send current draft message and kill it."
  (interactive)
  (wl-draft-send t))

If I have more than one window open, everything works as expected, the
message is sent (or the draft is killed), the window and its buffer are
removed.  The problem occurs only if only one window is open.

I haven't been able to track down where delete-window is called in these
functions.

I started having this problem after updating to wanderlust-20190623.1332.

I don't think it's due to anything in my config.  Does anyone have any
suggestions?

All the best,
Greg