ATSPI: GtkStackPage has no parent

When trying to iterate up through the parents of an Accessible, I noticed that in gtk4 applications this iteration stops too soon due to one get_parent() call returning NULL unexpectedly. So far I've noticed this only in GtkStackPages.

Here is a small python script to show the issue with gtk4-widget-factory when focusing different widgets: iterate_parents.py

When focusing the combo box in the first page (or any other widget on that page), the output is:

begin
'GtkEntry' is a 'text'
'' is a 'panel'
'GtkComboBoxText' is a 'combo box'
'' is a 'panel'
'' is a 'panel'
'' is a 'panel'
'Page _1' is a 'panel'
end

However when focusing one of the stack switcher buttons or the menu button I get:

begin
'Page _1' is a 'page tab'
'GtkStackSwitcher' is a 'page tab list'
'' is a 'panel'
'' is a 'panel'
'' is a 'panel'
'GTK Widget Factory' is a 'frame'
'gtk4-widget-factory' is a 'application'
'main' is a 'desktop frame'
end