your line raising the error is here:
comment = Comment.objects.get(pk=comment_id)
you try to access a non-existing comment.
from django.shortcuts import get_object_or_404
comment = get_object_or_404(Comment, pk=comment_id)
Instead of having an error on your server, your user will get a 404 meaning that he tries to access a non existing resource.
Ok up to here I suppose you are aware of this.
Some users (and I'm part of them) let tabs running for long time, if users are authorized to delete data, it may happens. A 404 error may be a better error to handle a deleted resource error than sending an email to the admin.
Other users go to addresses from their history, (same if data have been deleted since it may happens).