crash in gtk3widgets.py: AttributeError: 'NoneType' object has no attribute 'append'

The Ubuntu tracker contains some crash reports with the following traceback:

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/aptdaemon/gtk3widgets.py", line 522, in _on_download_changed
    iter = self._download_map[uri]
KeyError: 'http://pl.archive.ubuntu.com/ubuntu/pool/universe/w/wxwidgets2.8/wx2.8-i18n_2.8.12.1-14ubuntu1.1_all.deb'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/aptdaemon/gtk3widgets.py", line 525, in _on_download_changed
    iter = model.append((text, progress, uri))
AttributeError: 'NoneType' object has no attribute 'append'

These are unfortunately consolidated with the error report for bug 1024590, because the initial portion of the Traceback is the same. Regardless, it appears that model can be None in some cases.

Here is an example of the new crash:

https://errors.ubuntu.com/oops/380e4bce-7793-11e3-932c-e4115b0f8a4a

This is occurring with aptdaemon version 1.1.1-0ubuntu4 from saucy-updates and presumably with aptdaemon version 1.1.1-1ubuntu1 from trusty.