Whenever I try to add a widget to GtkActionBar, either by selecting a widget and clicking on the bar UI, or by right-clicking on it in the Inspector, I get an error dialog reporting "Widgets of type Action Bar need placeholders to add children".

If I manually add children to a GtkActionBar by editing a UI file, then load the file in Glade, make a trivial change, and save it, the children are removed from the file.