bug#4790: 23.1.50; Buffer switching with nXHTML

Please write in English if possible, because the Emacs maintainers
usually do not have translators to read other languages for them.

Your bug report will be posted to the [email protected] mailing list.

Please describe exactly what actions triggered the bug
and the precise symptoms of the bug:

When having an buffer open in xhtml mode iswtichbb stops working, when I
try to use it I get the following 

 Wrong type argument: stringp, (sgml-xml-mode "XHTML" "HTML")

In the minibuffer

Steps to reproduce

1) Use iswitchbb
2) Open an XHTML file
3) Try to switch to it using C-x b

If Emacs crashed, and you have the Emacs process in the gdb debugger,
please include the output from the following gdb commands:
    `bt full' and `xbacktrace'.
If you would like to further debug the crash, please read the file
/users/simonhj/local/share/emacs/23.1.50/etc/DEBUG for instructions.


In GNU Emacs 23.1.50.1 (i686-pc-linux-gnu, X toolkit, Xaw3d scroll bars)
 of 2009-09-21 on veloce
Windowing system distributor `The X.Org Foundation', version 11.0.70101000
configured using `configure  '--with-x-toolkit=athena' '--with-x' '--prefix=/users/simonhj/local' '--program-suffix=23''

Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: nil
  value of $LC_CTYPE: da_DK
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: nil
  value of $LANG: C
  value of $XMODIFIERS: @im=none
  locale-coding-system: iso-latin-1-unix
  default enable-multibyte-characters: t

Major mode: Org

Minor modes in effect:
  TeX-PDF-mode: t
  diff-auto-refine-mode: t
  show-paren-mode: t
  iswitchb-mode: t
  change-cursor-mode: t
  global-hi-lock-mode: t
  hi-lock-mode: t
  recentf-mode: t
  cua-mode: t
  global-hl-line-mode: t
  partial-completion-mode: t
  delete-selection-mode: t
  tooltip-mode: t
  mouse-wheel-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  global-auto-composition-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t
  abbrev-mode: t

Recent input:
<right> <right> <right> <right> <right> <right> <right> 
<C-backspace> <C-backspace> <C-backspace> B e t t e 
r SPC d e s c r i p t i o n ? C-x C-s <down> <down> 
<down> <down> <down> <down> <down> <down> <down> <down> 
<down> <down> <down> <down> <down> <down> <down> <down> 
<down> C-x v v C-c C-c <switch-frame> <switch-frame> 
<switch-frame> <switch-frame> <switch-frame> q <up> 
<up> <up> <up> <up> <up> <up> <up> <up> <up> <up> <up> 
<up> <up> <down> <down> <down> <down> <down> <down> 
<down> <down> <down> <down> <down> <down> <down> <down> 
<down> <down> <down> <down> <down> <down> <down> <down> 
<down> <down> <down> <down> <down> <down> <down> <down> 
<down> <down> <down> <down> <down> <down> <down> <down> 
<down> <down> <down> <down> <down> <down> <down> <down> 
<down> <down-mouse-1> <mouse-1> <backspace> <backspace> 
<backspace> <down> <up> C-x C-s <up> <up> C-c C-e h 
<up> <up> <up> <up> <left> <down> <down> <backspace> 
<backspace> <up> <down> <down> <down> <down> <down> 
<backspace> <up> <up> <up> <up> <up> <up> <up> <up> 
<up> <up> <up> <up> <backspace> C-x C-s C-c C-e h <switch-frame> 
<switch-frame> <switch-frame> <switch-frame> <help-echo> 
<help-echo> <help-echo> <switch-frame> <switch-frame> 
<down-mouse-1> <mouse-1> <backspace> <backspace> <backspace> 
<backspace> <delete> <delete> <delete> <insert> C-x 
C-s C-c C-e h <right> <backspace> <right> <backspace> 
C-x C-s M-q C-z <down> <down> <down> <down> <down> 
<down> <down> <down> <down> <down> <down> <down> <down> 
<down> <down> <down> <down> <down> <down> <down> <down> 
<down> <down> <down> <down> <down> <down> <down> <down> 
<down> <down> <down> <down> <down> <down> <down> <down> 
<down> <up> <up> <up> <up> <up> <up> <up> <up> <up> 
<up> <up> <up> <up> <up> <up> <up> <up> <up> <up> <switch-frame> 
<switch-frame> <switch-frame> <switch-frame> <switch-frame> 
<switch-frame> <switch-frame> <switch-frame> C-x C-f 
C-g C-x b C-x C-b <down> <down> <down> <down> <down> 
<down> <down> <down> <down> <up> <down> <up> C-g M-x 
r e <tab> p <tab> <tab> o r <tab> b <tab> <return>

Recent messages:
Select command: 
Exporting... [3 times]
Using vacuous schema [2 times]
Saving file /users/simonhj/web/TaPL/index.html...
Wrote /users/simonhj/web/TaPL/index.html
HTML export done, pushed to kill ring and clipboard
Saving file /users/simonhj/web/TaPL/index.org...
Wrote /users/simonhj/web/TaPL/index.org
Undo!
Quit
Wrong type argument: stringp, (sgml-xml-mode "XHTML" "HTML")

Load-path shadows:
/users/simonhj/elisp/nxml/related/css-mode hides ~/elisp/css-mode
~/elisp/slime/tree-widget hides /users/simonhj/local/share/emacs/23.1.50/lisp/tree-widget
~/elisp/linum hides /users/simonhj/local/share/emacs/23.1.50/lisp/linum
~/elisp/rst hides /users/simonhj/local/share/emacs/23.1.50/lisp/textmodes/rst
/users/simonhj/elisp/nxml/related/css-mode hides /users/simonhj/local/share/emacs/23.1.50/lisp/textmodes/css-mode
/users/simonhj/elisp/nxml-mode/rng-uri hides /users/simonhj/local/share/emacs/23.1.50/lisp/nxml/rng-uri
/users/simonhj/elisp/nxml-mode/nxml-glyph hides /users/simonhj/local/share/emacs/23.1.50/lisp/nxml/nxml-glyph
/users/simonhj/elisp/nxml-mode/nxml-enc hides /users/simonhj/local/share/emacs/23.1.50/lisp/nxml/nxml-enc
/users/simonhj/elisp/nxml-mode/xsd-regexp hides /users/simonhj/local/share/emacs/23.1.50/lisp/nxml/xsd-regexp
/users/simonhj/elisp/nxml-mode/xmltok hides /users/simonhj/local/share/emacs/23.1.50/lisp/nxml/xmltok
/users/simonhj/elisp/nxml-mode/rng-xsd hides /users/simonhj/local/share/emacs/23.1.50/lisp/nxml/rng-xsd
/users/simonhj/elisp/nxml-mode/rng-valid hides /users/simonhj/local/share/emacs/23.1.50/lisp/nxml/rng-valid
/users/simonhj/elisp/nxml-mode/rng-util hides /users/simonhj/local/share/emacs/23.1.50/lisp/nxml/rng-util
/users/simonhj/elisp/nxml-mode/rng-pttrn hides /users/simonhj/local/share/emacs/23.1.50/lisp/nxml/rng-pttrn
/users/simonhj/elisp/nxml-mode/rng-parse hides /users/simonhj/local/share/emacs/23.1.50/lisp/nxml/rng-parse
/users/simonhj/elisp/nxml-mode/rng-nxml hides /users/simonhj/local/share/emacs/23.1.50/lisp/nxml/rng-nxml
/users/simonhj/elisp/nxml-mode/rng-match hides /users/simonhj/local/share/emacs/23.1.50/lisp/nxml/rng-match
/users/simonhj/elisp/nxml-mode/rng-maint hides /users/simonhj/local/share/emacs/23.1.50/lisp/nxml/rng-maint
/users/simonhj/elisp/nxml-mode/rng-loc hides /users/simonhj/local/share/emacs/23.1.50/lisp/nxml/rng-loc
/users/simonhj/elisp/nxml-mode/rng-dt hides /users/simonhj/local/share/emacs/23.1.50/lisp/nxml/rng-dt
/users/simonhj/elisp/nxml-mode/rng-cmpct hides /users/simonhj/local/share/emacs/23.1.50/lisp/nxml/rng-cmpct
/users/simonhj/elisp/nxml-mode/nxml-util hides /users/simonhj/local/share/emacs/23.1.50/lisp/nxml/nxml-util
/users/simonhj/elisp/nxml-mode/nxml-uchnm hides /users/simonhj/local/share/emacs/23.1.50/lisp/nxml/nxml-uchnm
/users/simonhj/elisp/nxml-mode/nxml-rap hides /users/simonhj/local/share/emacs/23.1.50/lisp/nxml/nxml-rap
/users/simonhj/elisp/nxml-mode/nxml-parse hides /users/simonhj/local/share/emacs/23.1.50/lisp/nxml/nxml-parse
/users/simonhj/elisp/nxml-mode/nxml-outln hides /users/simonhj/local/share/emacs/23.1.50/lisp/nxml/nxml-outln
/users/simonhj/elisp/nxml-mode/nxml-ns hides /users/simonhj/local/share/emacs/23.1.50/lisp/nxml/nxml-ns
/users/simonhj/elisp/nxml-mode/nxml-mode hides /users/simonhj/local/share/emacs/23.1.50/lisp/nxml/nxml-mode
/users/simonhj/elisp/nxml-mode/nxml-maint hides /users/simonhj/local/share/emacs/23.1.50/lisp/nxml/nxml-maint