Comment 0 for bug 1799161

=== Environment ===
OS: Xenial 16.04.4, Linux BLRKECROSINF32 4.4.0-116-generic #140-Ubuntu SMP Mon Feb 12 21:23:04 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
MAAS version: 2.3.5
MAAS packages installed: https://pastebin.canonical.com/p/PtwDfKQDP4/

=== Problem summary ===

While deploying some random Juju bundle (in this case it was "magpie"), Juju is not able to get a machine from MAAS: https://pastebin.canonical.com/p/sWSfrFyGxk/
In the same time, MAAS regiond log contains following records:

2018-10-19 13:34:28 maasserver: [error] Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/maasserver/api/machines.py", line 561, in deploy
    get_curtin_merged_config(machine)
  File "/usr/lib/python3/dist-packages/maasserver/preseed.py", line 399, in get_curtin_merged_config
    yaml_config = get_curtin_yaml_config(node)
  File "/usr/lib/python3/dist-packages/maasserver/preseed.py", line 387, in get_curtin_yaml_config
    storage_config = compose_curtin_storage_config(node)
  File "/usr/lib/python3/dist-packages/maasserver/preseed_storage.py", line 640, in compose_curtin_storage_config
    return [generator.generate()]
  File "/usr/lib/python3/dist-packages/maasserver/preseed_storage.py", line 68, in generate
    self._generate_bcache_operations()
  File "/usr/lib/python3/dist-packages/maasserver/preseed_storage.py", line 505, in _generate_bcache_operations
    self._generate_bcache_operation(filesystem_group)
  File "/usr/lib/python3/dist-packages/maasserver/preseed_storage.py", line 514, in _generate_bcache_operation
    "backing_device": filesystem_group.get_bcache_backing_filesystem(
AttributeError: 'NoneType' object has no attribute 'get_parent'

Full log: https://pastebin.canonical.com/p/RTrPCDkQ59/

Manual deployment via GUI leads to same error messages in logs - "Logs > Installation output" shows only "System is booting", while similar log records appears in regiond log.

Output of "maas admin machine read ....": https://pastebin.canonical.com/p/qB8s5m5HSz/