Thank you for your bug-report. Fixed in revision 71.
But the main problem is that we can not mix Gtk2 and Gtk3 on 18.04 LTS. I have the following errors on bionic VM (while trying to run mousetweaks_3.99.99-really3.0.0~71~ubuntu16.04.1_amd64.deb on it):
mate@bionic:~/Downloads$ dwell-click-applet
(dwell-click-applet:22372): Gtk-ERROR **: 13:40:19.634: GTK+ 2.x symbols detected. Using GTK+ 2.x and GTK+ 3 in the same process is not supported
Trace/breakpoint trap (core dumped)
mate@bionic:~/Downloads$ pointer-capture-applet
(pointer-capture-applet:22380): Gtk-ERROR **: 13:40:51.316: GTK+ 2.x symbols detected. Using GTK+ 2.x and GTK+ 3 in the same process is not supported
Trace/breakpoint trap (core dumped)
So here we need some non trivial (for me) patching from @vkareh