[O] org-drill problem: questions are blank

[Peter Westlake]

>From time to time Org-drill shows me a blank window instead of a
question. The frame, mode line and prompt to answer are visible, but
there's nothing in the window at all. As a workaround I can type "t" to
edit the tags, and the question will appear.

A sample file is enclosed. It's a new file without any of the org-drill
property drawers just to keep the example short and to avoid having to
wait for the question to be scheduled to be shown again, but the problem
*does* still happen when the drawers are present.

To reproduce the bug, run org-drill in drillbug.org; the question "MC"
gives a blank screen.

I'm using org-drill with the latest version of Org-mode, git commit
2f58e3c011ec84f621905332d8df68b83da580d9, on Emacs 24.3.1, 2014-09-30.

Peter.