bug#42589: 26.3; "Wrong type argument: stringp, nil"

[Drew Adams]

emacs -Q

`M-x load-library dired-x'

`C-x d' for a dir that has a subdir

With cursor on a subdir line, `i', to insert its listing.

With cursor on the inserted-subdir heading line: `!'.

You get that raw error message.

`!' is not intended for directory heading lines.  But the error
raised shouldn't be one you see.


In GNU Emacs 26.3 (build 1, x86_64-w64-mingw32)
 of 2019-08-29
Repository revision: 96dd0196c28bc36779584e47fffcca433c9309cd
Windowing system distributor `Microsoft Corp.', version 10.0.18362
Configured using:
 `configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install 'CFLAGS=-O2 -static -g3''