GtkBuilder cannot create an object whose type hasn't been ensured
As stated in https://gitlab.gnome.org/GNOME/gtk/-/blob/gtk-3-24/gtk/glade/README.glade#L14, GtkBuilder
fails like this:
(org.example.App:2): Gtk-CRITICAL **: 20:43:17.878: Error building template class 'TesttWindow' for an instance of type 'TesttWindow': .:9:1 Invalid object type 'HdyColumn'
If the type in question hasn't been loaded yet.
To prevent that, one must manually ensure it like here: https://gitlab.gnome.org/GNOME/gtk/-/blob/gtk-3-24/gtk/gtkfilechooserwidget.c#L8883-8887
I was told that's not the expected behavior and it's supposed to just work.