[O] Org beamer async export fails

[Jeremie Juste]

Hello,

I noticed that async export using org-beamer-export fails. For instance

<C-c C-e C-a l P> on test.org (see below)

yield

<<Unknown "nil" back-end: Aborting export>>



I'm using Org mode version 9.1.13

Best regards,
Jeremie

content of test.org

#+TITLE: Example Presentation
#+AUTHOR: Carsten Dominik
#+OPTIONS: H:2 toc:t num:t
#+LATEX_CLASS: beamer
#+LATEX_CLASS_OPTIONS: [presentation]
#+BEAMER_THEME: Madrid
#+COLUMNS: %45ITEM %10BEAMER_ENV(Env) %10BEAMER_ACT(Act) %4BEAMER_COL(Col) 
%8BEAMER_OPT(Opt)

* This is the first structural section test

** Frame 1
*** Thanks to Eric Fraga                                           :B_block:
    :PROPERTIES:
    :BEAMER_COL: 0.48
    :BEAMER_ENV: block
    :END:
    for the first viable Beamer setup in Org
*** Thanks to everyone else                                        :B_block:
    :PROPERTIES:
    :BEAMER_COL: 0.48
    :BEAMER_ACT: <2->
    :BEAMER_ENV: block
    :END:
    for contributing to the discussion
**** This will be formatted as a beamer note                       :B_note:
     :PROPERTIES:
     :BEAMER_env: note
     :END:
** Frame 2 (where we will not use columns)
*** Request
    Please test this stuff!