Since at least Emacs 24.4, the commands gnus-summary-widget-forward and
gnus-summary-widget-backward have been signalling an error in multiframe
configurations:

0. HOME=$(mktemp -d) emacs -Q
1. (progn (setq debug-on-error t
                gnus-select-method '(nntp "news.gwene.org"))
          (gnus-add-configuration
           '(article (frame 1.0
                            (summary 1.0 point frame-focus)
                            (article 1.0)))))
2. C-j
3. M-x gnus RET
4. s          [session only]
5. ^          [gnus-group-enter-server-mode]
6. n          [goto nntp:news.gwene.org]
7. RET        [gnus-server-read-server]
8. C-s gmane.emacs.devel RET
9. u          [gnus-browse-unsubscribe-current-group]
10. q         [gnus-browse-exit]
11. q         [gnus-server-exit]
12. RET 1 RET [gnus-group-select-group]
13. TAB       [gnus-summary-widget-forward]

Debugger entered--Lisp error: (wrong-type-argument window-live-p nil)
  select-window(nil)
  gnus-summary-widget-forward(1)
  funcall-interactively(gnus-summary-widget-forward 1)
  call-interactively(gnus-summary-widget-forward nil nil)
  command-execute(gnus-summary-widget-forward)

Is it okay to push the attached fix, which additionally simplifies the
subroutine gnus-get-buffer-window used by the two commands?

Thanks,

-- 
Basil

Gnus v5.13
In GNU Emacs 27.0.50 (build 14, x86_64-pc-linux-gnu, X toolkit, Xaw3d scroll bars)
 of 2019-05-04 built on thunk
Repository revision: b5376e0ebd7b62f00c1b64aef87cd8d96cfbc71a
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12003000
System Description: Debian GNU/Linux buster/sid