Fixed.
Any idea why i cannot use log.error() here ?
```
  File "./picard/util/cdrom.py", line 34, in <module>
    log.error(traceback.format_exc())
  File "./picard/log.py", line 75, in error
    thread.proxy_to_main(_message, "E:", message, args, kwargs)
  File "./picard/util/thread.py", line 110, in proxy_to_main
    ThreadPool.instance.call_from_thread(handler, *args, **kwargs)
AttributeError: 'NoneType' object has no attribute 'call_from_thread'
```