The glade-3.8.x series is the last version supporting Gtk+2. glade > 3.8.x targets Gtk+3. Ubuntu provides the glade-gtk2
package which packages the 3.8.x version, whereas the glade
package packages the latest, Gtk+3 only version.
So if you are targeting Gtk+2, you need to install glade-gtk2
and use that version of glade.
Note: similarly on Fedora, the glade
package ships the Gtk+3 version, whereas the glade3
(not really an optimal name) ships the Gtk+2 version.