GLib-GObject-WARNING **:cannot create instance of abstract (non-instantiatable) type `GtkBox'
When using Visual Ruby and/or Glade to design a Ruby-GNOME2 user interface, the reason is that the latest version of Glade (3.10) has dropped support for Gtk::VBox and Gtk::HBox - it replaces them both with Gtk::Box. This breaks Gtk::Builder. The solution is to use Glade 3.08, which can be installed (for Ubuntu, and Ubuntu based distributions such as Linux Mint) using:
sudo apt-get install glade-gtk2=3.8.0-0ubuntu3
To find the correct version/package name for your distribution, use:
apt-cache showpkg glade-gtk2
and find version 3.8.0.
If using Visual Ruby, be sure to update the Glade Command Line setting by going to Tools -> Settings and entering
in the Glade Command Line box.