Comment 11 for bug 1796981

Re-reading the commit message of the fix, indeed it's probably that the method will return None if the guest is gone during the evacuate.

We handle InstanceNotFound in the block_stats method:

https://github.com/openstack/nova/blob/6bf11e1dc14afad78b11d980c2544a3dc41579ff/nova/virt/libvirt/driver.py#L6364-L6381

But this is probably a race where we are able to get a guest reference to the instance on the host _before_ it's gone (so we don't get InstanceNotFound) but by the time we call dev.blockStats(), the guest is gone, so we get None returned.

Anyway, the merged patch should fix the issue.