When you try to load the UI definition using Gtk.Builder.add_from_file, then it complains about those two packing tags. Commenting them out removes the issues. But of course, that means a lot of manual editing of XML-files all the time, so it's a very annoying bug.
This may or may not be a bug in Glade, but I'm unable to find out. The problem is with InfoBar. Glade adds
<child> child=" content_ area">
<packing>
</ packing>
<object class="GtkInfoBar" id="infobar1">
<child internal-
</child>
<child internal- child=" action_ area">
<packing>
</ packing>
</child>
When you try to load the UI definition using Gtk.Builder. add_from_ file, then it complains about those two packing tags. Commenting them out removes the issues. But of course, that means a lot of manual editing of XML-files all the time, so it's a very annoying bug.