switchover action can fail

[Stuart Bishop]

2017-11-28 11:07:57 DEBUG replication-relation-changed Traceback (most recent call last):
2017-11-28 11:07:57 DEBUG replication-relation-changed File "/var/lib/juju/agents/unit-postgresql-2/charm/hooks/replication-relation-changed", line 19, in <module>
2017-11-28 11:07:57 DEBUG replication-relation-changed main()
2017-11-28 11:07:57 DEBUG replication-relation-changed File "/usr/local/lib/python3.5/dist-packages/charms/reactive/__init__.py", line 78, in main
2017-11-28 11:07:57 DEBUG replication-relation-changed bus.dispatch()
2017-11-28 11:07:57 DEBUG replication-relation-changed File "/usr/local/lib/python3.5/dist-packages/charms/reactive/bus.py", line 423, in dispatch
2017-11-28 11:07:57 DEBUG replication-relation-changed _invoke(other_handlers)
2017-11-28 11:07:57 DEBUG replication-relation-changed File "/usr/local/lib/python3.5/dist-packages/charms/reactive/bus.py", line 406, in _invoke
2017-11-28 11:07:57 DEBUG replication-relation-changed handler.invoke()
2017-11-28 11:07:57 DEBUG replication-relation-changed File "/usr/local/lib/python3.5/dist-packages/charms/reactive/bus.py", line 280, in invoke
2017-11-28 11:07:57 DEBUG replication-relation-changed self._action(*args)
2017-11-28 11:07:57 DEBUG replication-relation-changed File "/var/lib/juju/agents/unit-postgresql-2/charm/reactive/postgresql/replication.py", line 617, in drain_master_and_promote_anointed
2017-11-28 11:07:57 DEBUG replication-relation-changed remote_offset = postgresql.wal_received_offset(remote_con)
2017-11-28 11:07:57 DEBUG replication-relation-changed File "/var/lib/juju/agents/unit-postgresql-2/charm/reactive/postgresql/postgresql.py", line 695, in wal_received_offset
2017-11-28 11:07:57 DEBUG replication-relation-changed return wal_location_to_bytes(xlog_received)
2017-11-28 11:07:57 DEBUG replication-relation-changed File "/var/lib/juju/agents/unit-postgresql-2/charm/reactive/postgresql/postgresql.py", line 726, in wal_location_to_bytes
2017-11-28 11:07:57 DEBUG replication-relation-changed logid, offset = wal_location.split('/')
2017-11-28 11:07:57 DEBUG replication-relation-changed AttributeError: 'NoneType' object has no attribute 'split'