Strange problems when exporting asynchronously in beamer mode

[Julien Cubizolles <j.cubizolles@free.fr>]

I recently upgraded to the latest git version and every asynchronous
export to pdf in beamer mode fails whereas the asynchronous mode has no
problem. I didn't change anything in my setup, and it was working fine
before.

With the following Org file (the first lines are to ensure that the
latest git version of Org is used)

--8<---------------cut here---------------start------------->8---
#+BEGIN_SRC emacs-lisp :exports none :results none
  (add-to-list 'load-path (expand-file-name "~/info/emacs/org-mode/lisp"))
  (add-to-list 'load-path (expand-file-name "~/info/emacs/org-mode/contrib/lisp"))
  (setq org-export-async-init-file "/home/wilk/configuration/elisp/org-export-async-init.el")
  (org-reload)
  (setq org-export-async-debug t)
#+END_SRC

* Test
essai
--8<---------------cut here---------------end--------------->8---

Calling (in emacs -Q) an asynchronous export to Tex for Beamer (C-c C-e
l b) gives the following error
--8<---------------cut here---------------start------------->8---
Unknown "nil" back-end: Aborting export
--8<---------------cut here---------------end--------------->8---

There is no error for an export to TeX in LateX mode (C-c C-e l l), of
if the export to TeX in beamer mode isn't asynchronous.

The contents of the
"/home/wilk/configuration/elisp/org-export-async-init-test.el" file are:

--8<---------------cut here---------------start------------->8---
;;~/.emacs -*- mode: lisp-*-
(setq org-export-async-debug t)

(add-to-list 'load-path (expand-file-name "~/info/emacs/org-mode/lisp"))
(add-to-list 'load-path (expand-file-name  "~/info/emacs/org-mode/contrib/lisp"))

(require 'org)
(require 'ox)
--8<---------------cut here---------------end--------------->8---

Any idea what went wrong ?

Julien.