The fatal error appears because the item in the basket doesn't have a corresponding subscription, as the error log also tells.
Are you sure you are not getting an old basket where you deleted the subscription ?
To see if it's that, unpublish Promotions, and try again. If you have a warning at top that you are seeing previous basket, it's that, click Cancel order and subscriptions, then re-publish promotions, and it should work.
I'm writing down the fatal error, and will add a test-case so it can log a more clear error.
#2781