[Gtk-sharp-list] Problem with Gtk VALID_ITER failing, even after I check it with IterIsValid (TreeIter iter)

[Matthew Pirocchi]

I have a convenience function in my TreeView that looks something like this:

Card GetCardFromPath (TreePath path)
{
TreeIter iter;
if (path == null || !Model.GetIter (out iter, path))
return null;
 if ((Model as TreeModelSort).IterIsValid (iter))
return (Card) Model.GetValue (iter, 0);
 return null;
}

Most of the time this works without any errors. But when it is called
directly after the Model is changed, line 8 gives me these Gtk runtime
errors:
[Fatal 16:53:02.448] [Gtk] gtk_list_store_get_value: assertion `VALID_ITER
(iter, list_store)' failed
[Fatal 16:53:02.449] [GLib-GObject] g_value_unset: assertion `G_IS_VALUE
(value)' failed

As far as I can tell, I shouldn't even need to check IterIsValid, because
I'm already checking the return value of Model.GetIter. Even so, how can
VALID_ITER fail in a function that only gets called if IterIsValid returns
true?

If it makes a difference, the Model is a TreeModelSort, which sorts a
TreeModelFilter, which filters a ListStore. The whole class is here:
http://bazaar.launchpad.net/~matthew-pirocchi/cardinal/trunk/annotate/head:/Cardinal.Interface/src/CardTreeView.cs#L158

The error occurs when GetCardFromPath is called from HandleSelectionChanged
when multiple rows have just been removed from the ListStore. It doesn't
seem to prevent anything from working properly, but having a cascade of
errors whenever I remove multiple rows isn't really ideal.
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