Try launching:
bacongui-gtk -gtk3
Then, while in the ide, under the "options" menu select "update syntax file..." and when that's done, in the same "options" menu, click on "Select Theme" and choose any of the themes.
After this, when you relaunch, you should no longer see any of the gtksourceview warnings. You will still see the message about not finding Gtk2, ignore it, since it will use Gtk3 which is the default in regular Ubuntu.