OpenEdx Exams - System Check don't work


I’m working with Proctored Exam, im following step in:
Im stuck at step 2, when i click button “Start System Check”, im redirect to:
It’s 404 page and it said :“Position None is not an integer!”.
My setting:

PROCTORING_BACKENDS {'DEFAULT': 'null', 'null': {}}

Im using Docker devstack, I tried to download RPnow from website but cannot validate my course’s exam code.
Im new, hope some help.
Thanks everyone !