dropbox icon not showing

cf: https://askubuntu.com/questions/1029225/how-to-fix-broken-nautilus-dropbox-icon-in-ubuntu-18-04-gnome-flashback-in-top-p

1. dropbox stop
2. gedit ~/.config/autostart/dropbox.desktop
   replace Exec=dropbox start -i with
   Exec=env XDG_CURRENT_DESKTOP=Unity dropbox start
3. sudo gedit /usr/share/applications/dropbox.desktop
   replace Exec=dropbox start -i with
   Exec=env XDG_CURRENT_DESKTOP=Unity dropbox start
4. click Dropbox in dash to start it.
5. Done.