The plugin doesn't even use it. It tries to get the gui_name into a variable, but never actually uses that variable.
I really hope that wasn't one of the changes I've made.
@read-more-books: I have an update to the plugin that I've been to lazy to release. I'll try and get it out this week. In the meantime, you will need to disable or uninstall the Annotations plugin. Removing it from the toolbar might also work and can be done without restarting calibre if you are swapping between the devices a lot.
@Kovid: Shouldn't all devices have a gui_name?