bug#36288: 27.0.50; Cannot use menu-set-font

[Robert Pluim]

emacs -Q
M-x menu-set-font
select Ubuntu Mono Regular =>

"set-face-attribute: Font not available: #<font-spec xft nil Ubuntu\ Mono nil 
nil normal normal nil 10.0 nil nil nil ((:name . "Ubuntu Mono"))>"

I can select Ubuntu Mono using M-x customize-face. Why does
menu-set-font try to use an xft spec?

(frame-parameter nil 'font-backend) =>
(xfthb x)



In GNU Emacs 27.0.50 (build 4, x86_64-pc-linux-gnu, GTK+ Version 3.22.30)
 of 2019-06-19
Repository revision: 50c5d5621cb5e6d7c86829ac4b776d81e47b2189
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.11804000
System Description: Ubuntu 18.04.2 LTS

Configured features:
XPM JPEG TIFF GIF PNG RSVG SOUND GPM DBUS GSETTINGS GLIB NOTIFY INOTIFY
ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE HARFBUZZ M17N_FLT LIBOTF XFT ZLIB
TOOLKIT_SCROLL_BARS GTK3 X11 XDBE XIM THREADS PDUMPER LCMS2 GMP