Comment 1 for bug 1374243

Qt can only be used in the main thread of the program, bulk metadata
runs in its own worker thread, thefre you have the issue.

What needs to be done is, if is_ok_to_use_qt() returns False, you have
to launch a worker process using fork_job() and render the cover in
that.

If you wish to create a patch, fell free, otherwise I will get around
to it when I have some time.