U++ unfortunately doesn't work with GTK3 yet. The error you see is a result of API changes between GTK2 and GTK3. If there is GTK2 available for Mint, it should work (provided you set the includes back to the GTK2 paths).
Best regards,
Honza
Thank you Honza
In this case I will try on next days to uninstall libgtk3-dev and to install libgtk2-dev.