Comment 2 for bug 841713

From UsbCreatorLog.txt:
usb-creator 2011-09-05 11:24:06,212 (DEBUG) frontend.py:183: add_source: /home/mark/Downloads/lubuntu-11.10-beta1-desktop-amd64.iso
usb-creator 2011-09-05 11:24:06,216 (ERROR) usb-creator-gtk:79: Unhandled exception:
Traceback (most recent call last):
  File "/usr/bin/usb-creator-gtk", line 62, in <module>
    allow_system_internal=options.allow_system_internal)
  File "/usr/lib/python2.7/dist-packages/usbcreator/frontends/gtk/frontend.py", line 144, in __init__
    self.backend.add_image(os.path.join(download_dir, fname))
  File "/usr/lib/python2.7/dist-packages/usbcreator/backends/base/backend.py", line 51, in add_image
    self.source_added_cb(filename)
  File "/usr/lib/python2.7/dist-packages/usbcreator/frontends/gtk/frontend.py", line 185, in add_source
    force_selection=False)
  File "/usr/lib/python2.7/dist-packages/usbcreator/frontends/gtk/frontend.py", line 764, in _append_to_list_and_select
    treeview.set_cursor(model.get_path(new_iter), None, None)
  File "/usr/lib/python2.7/dist-packages/gi/types.py", line 43, in function
    return info.invoke(*args, **kwargs)
TypeError: Argument 3 does not allow None as a value