Hey there. I just ran into the same issue and fixed it by replacing an old (deprecated in BP 8.0.0) call to bp_nouveau_base_account_has_xprofile() with bp_nouveau_has_signup_xprofile_fields(true). The true is very important as it will initialize the global variable $profile_template, which is used by bp_profile_groups() (the function that is erroring). You’d have to change this in the overwritten buddypress register.php template, used by your theme. My template probably looks different from yours, so I can’t provide the specific lines that you’d have to make changes to. Good luck!
