I created a new "Moodleoverflow" activity. Filled in its name. Configured the settings under "Activity completion" as below picture. Clicked "save and return to course" button. Then it showed the error message.
Thanks for your bug report. You shouldn't be able to use the 'Require Grade' as activity completion setting if you didn't configure the grade options. This should be fixed now on the master branch :)
I created a new "Moodleoverflow" activity. Filled in its name. Configured the settings under "Activity completion" as below picture. Clicked "save and return to course" button. Then it showed the error message.