Traceback (most recent call last):
  File "/usr/bin/novacut-gtk", line 175, in on_thumbnail
    self.Renderer.Thumbnail(file_id, frames)
  File "/usr/lib/python3/dist-packages/dbus/proxies.py", line 70, in __call__
    return self._proxy_method(*args, **keywords)
  File "/usr/lib/python3/dist-packages/dbus/proxies.py", line 145, in __call__
    **keywords)
  File "/usr/lib/python3/dist-packages/dbus/connection.py", line 641, in call_blocking
    message.append(signature=signature, *args)
TypeError: an integer is required (got type NoneType)