Please try the new build, V.0.8.1C. I've removed the extension and tested it on a separate Windows 10 computer, it should be working correctly now.
Problem identified and patched, version 0.8.2 is now fully stable.
I use the new build, V.0.8.1c and got the same error when the first mission ended
___________________________________________
############################################################################################
FATAL ERROR in
action number 1
of Alarm Event for alarm 0
for object o_mission_end:
DoAdd :2: undefined value
at gml_Script_scr_test_save
############################################################################################
--------------------------------------------------------------------------------------------
stack frame is
gml_Script_scr_test_save (line -1)
gml_Object_o_mission_end_Alarm_0
Very sorry about that, I'm still not sure what's causing it. Can you try starting a new game? For some reason that fixes the problem for some people.
EDIT: The problem has been identified and a patch will go out today. Finally figured it out.
EDIT 2: Patched version now live, everything should be working correctly.