If you're getting that error there's something strange about the data being returned by nova volume (e.g. it finds an attachment but that attachment doesn't have an "instance" key as it should), however we could code that more defensively and have it fail more elegantly.
The relevant line is here: https:/ /github. com/openstack/ horizon/ blob/master/ horizon/ dashboards/ nova/instances_ and_volumes/ volumes/ tables. py#L96
If you're getting that error there's something strange about the data being returned by nova volume (e.g. it finds an attachment but that attachment doesn't have an "instance" key as it should), however we could code that more defensively and have it fail more elegantly.