Install unity-gtk3-module by entering the following commands in the terminal:
sudo apt update sudo apt install unity-gtk3-module
Description:
GtkMenuShell D-Bus exporter
This GTK+ 3 module exports GtkMenuShells over D-Bus.
Homepage: https://code.launchpad.net/unity-gtk-module
Version: 0.0.0+18.04.20171202-0ubuntu1
Section: universe/libs