After enabling Ex-Mortis on gedit 3.30.2 on Xubuntu 18.10, I see this error in the terminal:
Traceback (most recent call last):
File "/home/moi/.local/share/gedit/plugins/ex-mortis/__init__.py", line 108, in do_activate
menu_ext.append_menu_item(menu_item)
AttributeError: 'NoneType' object has no attribute 'append_menu_item'
I'm still able to access the Preferences window and check "Restore windows between sessions", but it doesn't work.