GtkListStore with None values

[Conscious User <conscioususer gmail com>]

Hi,

I'm working with a Gtk.ComboBox that uses a Gtk.ListStore
as a model and Gtk.CellRendererText as the renderer. The
model has two GdkRGBA elements that are added as the
renderer attributes "background-rgba" and
"foreground-rgba".

This was working with no warnings until today when, after
a PyGObject update, several warnings like "liststore.c:
unable to convert from (null) to GdkRGBA" started to appear.

So it seems that the model is not accepting None values
anymore. But I was using None to indicate that the cell
should use the default color (like override_color does).
What should I do now to indicate this?

Thanks for any help!