Fix a critical warning in notebook dnd with rgba colormaps
Fix a critical warning in notebook dnd with rgba colormaps

Pointed out by Nick Schermer in
https://bugzilla.gnome.org/show_bug.cgi?id=690788

Distrotech/gtk,johne53/MB3Gtk-2,johne53/MB3Gtk-2,Distrotech/gtk,johne53/MB3Gtk-2,johne53/MB3Gtk-2,Distrotech/gtk,Distrotech/gtk,johne53/MB3Gtk-2,Distrotech/gtk