Undercloud install fails with TypeError: 'NoneType' object is not subscriptable

Issue

• When running openstack undercloud install , it fails with :

Performing Heat stack create.. 
Exception: 'NoneType' object is not subscriptable
Traceback (most recent call last):
  File "/usr/lib/python3.6/site-packages/tripleoclient/v1/tripleo_deploy.py", line 1265, in _standalone_deploy
    parsed_args)
  File "/usr/lib/python3.6/site-packages/tripleoclient/v1/tripleo_deploy.py", line 828, in _deploy_tripleo_heat_templates
    stack_id = stack['stack']['id']
TypeError: 'NoneType' object is not subscriptable

2020-08-17 13:40:38.036 5545 INFO heat.api [-] Starting Heat REST API on 127.0.0.1:8006
2020-08-17 13:40:38.040 5545 WARNING oslo_config.cfg [-] Deprecated: Option "deferred_auth_method" from group "DEFAULT" is deprecated for removal (Stored password based deferred auth is broken when used with keystone v3 and is not supported.).  Its value may be silently ignored in the future.
2020-08-17 13:40:38.048 5545 INFO heat.common.wsgi [-] Starting single process server
2020-08-17 13:40:38.048 5545 INFO eventlet.wsgi.server [-] (5545) wsgi starting up on http://127.0.0.1:8006
2020-08-17 13:40:38.049 5545 INFO heat.engine.worker [-] Starting engine_worker (1.4) in engine 30f6bcf8-cc83-4cd1-9854-19640efab70d
(then no logs at all)
And then this error:
2020-08-17 13:40:52.244 5514 WARNING tripleoclient.v1.tripleo_deploy.Deploy [  ] ** Performing Heat stack create.. **
2020-08-17 13:40:52.821 5514 ERROR tripleoclient.v1.tripleo_deploy.Deploy [  ] Exception: 'NoneType' object is not subscriptable
2020-08-17 13:40:52.822 5514 ERROR tripleoclient.v1.tripleo_deploy.Deploy [  ] None