I don't see how this is a bug in the C++ bindings for GLib.
The error is not in ubiquity, but in the GTK+ CSS provider code, as the logs show:
Gtk:ERROR:/build/buildd/gtk+3.0-3.1.12/./gtk/gtkcssprovider.c:1275:gtk_css_scanner_new: assertion failed: (data[length] == 0)
I don't see how this is a bug in the C++ bindings for GLib.
The error is not in ubiquity, but in the GTK+ CSS provider code, as the logs show: /build/ buildd/ gtk+3.0- 3.1.12/ ./gtk/gtkcsspro vider.c: 1275:gtk_ css_scanner_ new: assertion failed: (data[length] == 0)
Gtk:ERROR: