Comment 3 for bug 1488475

I think it's failing as the output 'OpenSec' is None for APP-secgroup.template.yaml, before the nested stack is created. When you are creating a list it's creating a list like '[None]'.

You can work around this by changing the following in APP-secgroup.template.yaml:
......
  OpenSec:
    description: Open security group.
    value: [{ get_resource: APP-open-sec }]

and then in hot.yaml don't create a list

    APP-server01pair:
        type: APP::VmPair
        depends_on:
            - APPnetworking
        properties:
 .....
            eth0_sec_groups: {get_attr: [APPnetworking, OpenSec]}

At least this works with master. I'll check how it used to work in Juno.