bug#34451: 26.1; doc-view-toggle-display chokes if default major-mode is text-mode

[Roland Winkler]

Start `emacs -Q'
eval  (setq-default major-mode 'text-mode)
visit a pdf or postscript file
Type C-c C-c twice
doc-view-toggle-display: Wrong type argument: stringp, nil

The problem is that doc-view-toggle-display assumes that if the
major-mode is text-mode the user is viewing the document's text
contents.  But that's incorrect if the default value of major-mode
is text-mode so that this mode is also used for a "raw" pdf or
postscript file.


In GNU Emacs 26.1 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.18.9)
 of 2018-05-29 built on regnitz
Windowing system distributor 'The X.Org Foundation', version 11.0.11804000
System Description:     Ubuntu 16.04.5 LTS

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
text-mode
DocView: using cached files!
Type C-c C-c to toggle between editing or viewing the document. [2 times]
doc-view-toggle-display: Wrong type argument: stringp, nil

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GSETTINGS NOTIFY
ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB
TOOLKIT_SCROLL_BARS GTK3 X11 THREADS LCMS2

Important settings:
  value of $LC_COLLATE: C
  value of $LANG: en_US.utf8
  value of $XMODIFIERS: 
  locale-coding-system: utf-8-unix