Looks like you're just outside the {% if g.authenticated ... check from line 138, here (it closed on line 164), so you'd have to check again before using g.fas_user.
Suggestion:
{% if g.authenticated and (g.repo_committer or g.fas_user.username in g.repo.collaborators) %}
This PR enables collaborators to merge, while not restricting the people with committer access to merge the pull request.
Please refer to this issue for the relevant request/use-case reference.
Looks like you're just outside the
{% if g.authenticated ...
check from line 138, here (it closed on line 164), so you'd have to check again before usingg.fas_user
.Suggestion: