(1.3.10) Reproduced using milestones mod only:
Open Milestones settings
Select any entry and delete it
(i notice at this point the add/delete buttons are still enabled even though nothing is selected anymore)
Without selecting any other entry, add any new entry with the buttons. It appears at the top of the list.
Select that new entry with the checkbox to trigger the crash.
173.101 Error MainLoop.cpp:1284: Exception at tick 508: The mod Milestones (1.3.10) caused a non-recoverable error.
Please report this error to the mod author.
Error while running event Milestones::on_gui_click (ID 1)
Milestones/scripts/gui_settings_page.lua:262: attempt to index field 'milestones_settings_checkbox' (a nil value)
stack traceback:
Milestones/scripts/gui_settings_page.lua:262: in function 'update_checkbox_states'
Milestones/scripts/gui_settings_page.lua:312: in function 'select_setting'
Milestones/scripts/gui.lua:298: in function <Milestones/scripts/gui.lua:260>