[O] M-x org-publish in 8.3.1 results in error "org-check-agenda-file: Wrong type argument: stringp, nil"

[Omid]

Hello,

In Emacs 24.5.1, after updating to the latest Org mode in ELPA
(8.3.1-16-gf6aa53-elpa), org-publish results in the error

"org-check-agenda-file: Wrong type argument: stringp, nil"

on a project that used to publish without errors using the latest Org 8.2.

Here is the part of a backtrace of the error (using M-x
toggle-debug-on-error) that I believe is relevant

Debugger entered--Lisp error: (wrong-type-argument stringp nil)
  file-exists-p(nil)
  org-check-agenda-file(nil)
  byte-code("")
[file pos org-agenda-ignore-properties org-todo-keywords-for-agenda
org-todo-keywords-1 org-done-keywords-for-agenda bufferp
org-check-agenda-file org-get-agenda-file-buffer
org-set-regexps-and-options tags-only category
org-refresh-category-properties stats org-refresh-stats-properties
effort org-refresh-effort-properties appt org-refresh-properties
"APPT_WARNTIME" org-appt-warntime append copy-sequence delete-dups
org-uniquify-alist buffer-modified-p t nil ((byte-code " [modified
restore-buffer-modified-p nil] 2)) remove-text-properties
re-search-forward org-at-heading-p add-text-properties point-at-bol
org-end-of-subtree format "^\\* .*\\<%s\\>" match-data ((byte-code ""
[save-match-data-internal set-match-data evaporate] 3))
org-in-commented-heading-p 0 org-done-keywords
org-todo-keyword-alist-for-agenda org-todo-key-alist
org-tag-alist-for-agenda org-tag-alist org-tag-persistent-alist list res
org-group-tags ...] 5)
  org-agenda-prepare-buffers((nil))
  org-map-entries(#[nil "\300\301 !\207" [org-reduced-level
org-current-level] 2])
    call-interactively(org-publish record nil)
  command-execute(org-publish record)
  execute-extended-command(nil "org-publish")


I had to remove the arguments of byte-code() because they contained
special characters that couldn't be pasted in email.

Searching for this error, the only instance I could find was at

https://lists.gnu.org/archive/html/emacs-orgmode/2014-11/msg00343.html

reporting a the same error message, but the solution suggested there
is already implemented in 8.3 as a fix and is not applicable in this
case.

The relevant part of org.el is


(defun org-check-agenda-file (file)
  "Make sure FILE exists.  If not, ask user what to do."
  (when (not (file-exists-p file))
    (message "Non-existent agenda file %s.  [R]emove from list or [A]bort?"
             (abbreviate-file-name file))
    (let ((r (downcase (read-char-exclusive))))
      (cond
       ((equal r ?r)
        (org-remove-file file)
        (throw 'nextfile t))
       (t (user-error "Abort"))))))


How can I solve this problem?

Thanks,

Omid