Vim error with switchbuf="useopen" in vimdiff

[igrekster]

When setting g:wintabs_switchbuf="useopen", vimdiff file.a file.b results in a VIM error:

continuing in function wintabs#ui#get_tabline[2]..<SNR>53_get_bufline[5]..wintabs#memoize#call[5]..<SNR>53_get_bufline_non_memoized
Error detected while processing function <lambda>1[1]..<SNR>26_open_buffer_in:
line    2:
E16: Invalid range: 2wincmd w

And only one window visible.

Minimal .vimrc:

set nocompatible
silent! call plug#begin('~/.vim/plugged')
let g:wintabs_switchbuf="useopen"
Plug 'zefei/vim-wintabs'
call plug#end()

$ vim --version
VIM - Vi IMproved 8.1 (2018 May 18, compiled Aug 17 2019 13:55:59)
Included patches: 1-1846
Modified by Gentoo-8.1.1846
Compiled by portage@jentoo
Huge version without GUI.  Features included (+) or not (-):
+acl               -farsi             -mouse_sysmouse    -tag_any_white
+arabic            +file_in_path      +mouse_urxvt       -tcl
+autocmd           +find_in_path      +mouse_xterm       +termguicolors
+autochdir         +float             +multi_byte        +terminal
-autoservername    +folding           +multi_lang        +terminfo
-balloon_eval      -footer            -mzscheme          +termresponse
+balloon_eval_term +fork()            +netbeans_intg     +textobjects
-browse            -gettext           +num64             +textprop
++builtin_terms    -hangul_input      +packages          +timers
+byte_offset       +iconv             +path_extra        +title
+channel           +insert_expand     -perl              -toolbar
+cindent           +job               +persistent_undo   +user_commands
+clientserver      +jumplist          +postscript        +vartabs
+clipboard         +keymap            +printer           +vertsplit
+cmdline_compl     +lambda            +profile           +virtualedit
+cmdline_hist      +langmap           +python/dyn        +visual
+cmdline_info      +libcall           +python3/dyn       +visualextra
+comments          +linebreak         +quickfix          +viminfo
+conceal           +lispindent        +reltime           +vreplace
+cryptv            +listcmds          +rightleft         +wildignore
+cscope            +localmap          -ruby              +wildmenu
+cursorbind        +lua               +scrollbind        +windows
+cursorshape       +menu              +signs             +writebackup
+dialog_con        +mksession         +smartindent       +X11
+diff              +modify_fname      -sound             +xfontset
+digraphs          +mouse             +spell             -xim
-dnd               -mouseshape        +startuptime       -xpm
-ebcdic            +mouse_dec         +statusline        +xsmp_interact
+emacs_tags        -mouse_gpm         -sun_workshop      +xterm_clipboard
+eval              -mouse_jsbterm     +syntax            -xterm_save
+ex_extra          +mouse_netterm     +tag_binary
+extra_search      +mouse_sgr         -tag_old_static
   system vimrc file: "/etc/vim/vimrc"
     user vimrc file: "$HOME/.vimrc"
 2nd user vimrc file: "~/.vim/vimrc"
      user exrc file: "$HOME/.exrc"
       defaults file: "$VIMRUNTIME/defaults.vim"
  fall-back for $VIM: "/usr/share/vim"