Gtk3 override: Accessing TreeModel is broken

[vtrlx]

In the course of writing the Gtk4 override, I wanted to test the Gtk3 one against Gtk3 itself to ensure I didn't break anything, but it appears the move to GIRepository broke 2 Gtk3 tests. When trying to access a Gtk3 TreeModel instance using the Lua indexing operator, it appears not to do anything.

When tracing possible sources, I found that another of Gtk3's tests uses a TreeModel and accessing it works just fine. The only difference is in the use of the :append() method instead of the :insert() method, however the prior test failure is on the indexing operator and not the prior :get_value() call. It's quite puzzling.

[vtrlx]

I've found that the issue seems to stem from :_access_iter not getting set in the Gtk.TreeModel override. The only reason I can think for it not getting set is if somehow an instance of Gtk.TreeModel is being initialized before it's supposed to.

[vtrlx]

No, it's definitely getting set, but it seems to not be present when trying to retrieve it in :_access().