Conversion fails with trace in gstreamer.py

[MariusGarbea]

OpenSuSE Leap 42.2
SoundConverter 2.9.0-beta2

Conversion fails silently.
If launched from command prompt, one can see:
walking: 'file:///my_music_dircetory_to_convert'
Queue start: 14 tasks, 2 thread(s).
Queue done in 0.998s (14 tasks)
Traceback (most recent call last):
  File "/usr/lib64/soundconverter/python/soundconverter/ui.py", line 1323, in on_convert_button_clicked
    self.do_convert()
  File "/usr/lib64/soundconverter/python/soundconverter/ui.py", line 1309, in do_convert
    self.converter.start()
  File "/usr/lib64/soundconverter/python/soundconverter/gstreamer.py", line 804, in start
    self.waiting_tasks.sort(key=Converter.get_duration,reverse=True)
TypeError: unorderable types: NoneType() < NoneType()