Address traceback on relation broken

[sed-i]

Issue

When grafana-k8s is removed, we sometimes see

    data = self._charm.peers.data[self._charm.app].get(key, "")
AttributeError: 'NoneType' object has no attribute 'data'

Solution

Add a guard and return {} if the relation is None.

Fixes #365.

Context

Full traceback:

unit-graf-0: 18:46:29 WARNING unit.graf/0.grafana-dashboard-relation-broken   main(GrafanaCharm, use_juju_for_storage=True)
unit-graf-0: 18:46:29 ERROR unit.graf/0.juju-log grafana-dashboard:29: Uncaught exception while in charm code:
Traceback (most recent call last):
  File "./src/charm.py", line 1563, in <module>
    main(GrafanaCharm, use_juju_for_storage=True)
  File "/var/lib/juju/agents/unit-graf-0/charm/venv/ops/main.py", line 45, in main
    return _main.main(charm_class=charm_class, use_juju_for_storage=use_juju_for_storage)
  File "/var/lib/juju/agents/unit-graf-0/charm/venv/ops/_main.py", line 543, in main
    manager.run()
  File "/var/lib/juju/agents/unit-graf-0/charm/venv/ops/_main.py", line 529, in run
    self._emit()
  File "/var/lib/juju/agents/unit-graf-0/charm/venv/ops/_main.py", line 518, in _emit
    _emit_charm_event(self.charm, self.dispatcher.event_name, self._juju_context)
  File "/var/lib/juju/agents/unit-graf-0/charm/venv/ops/_main.py", line 134, in _emit_charm_event
    event_to_emit.emit(*args, **kwargs)
  File "/var/lib/juju/agents/unit-graf-0/charm/venv/ops/framework.py", line 347, in emit
    framework._emit(event)
  File "/var/lib/juju/agents/unit-graf-0/charm/venv/ops/framework.py", line 857, in _emit
    self._reemit(event_path)
  File "/var/lib/juju/agents/unit-graf-0/charm/venv/ops/framework.py", line 947, in _reemit
    custom_handler(event)
  File "/var/lib/juju/agents/unit-graf-0/charm/lib/charms/tempo_coordinator_k8s/v0/charm_tracing.py", line 735, in wrapped_function
    return callable(*args, **kwargs)  # type: ignore
  File "/var/lib/juju/agents/unit-graf-0/charm/lib/charms/grafana_k8s/v0/grafana_dashboard.py", line 1391, in _on_grafana_dashboard_relation_broken
    self._remove_all_dashboards_for_relation(event.relation)
  File "/var/lib/juju/agents/unit-graf-0/charm/lib/charms/tempo_coordinator_k8s/v0/charm_tracing.py", line 735, in wrapped_function
    return callable(*args, **kwargs)  # type: ignore
  File "/var/lib/juju/agents/unit-graf-0/charm/lib/charms/grafana_k8s/v0/grafana_dashboard.py", line 1525, in _remove_all_dashboards_for_relation
    if self._get_stored_dashboards(relation.id):
  File "/var/lib/juju/agents/unit-graf-0/charm/lib/charms/tempo_coordinator_k8s/v0/charm_tracing.py", line 735, in wrapped_function
    return callable(*args, **kwargs)  # type: ignore
  File "/var/lib/juju/agents/unit-graf-0/charm/lib/charms/grafana_k8s/v0/grafana_dashboard.py", line 1558, in _get_stored_dashboards
    return self.get_peer_data("dashboards").get(str(relation_id), {})
  File "/var/lib/juju/agents/unit-graf-0/charm/lib/charms/tempo_coordinator_k8s/v0/charm_tracing.py", line 735, in wrapped_function
    return callable(*args, **kwargs)  # type: ignore
  File "/var/lib/juju/agents/unit-graf-0/charm/lib/charms/grafana_k8s/v0/grafana_dashboard.py", line 1573, in get_peer_data
    data = self._charm.peers.data[self._charm.app].get(key, "")  # type: ignore[attr-defined]
AttributeError: 'NoneType' object has no attribute 'data'
unit-graf-0: 18:46:30 ERROR juju.worker.uniter.operation hook "grafana-dashboard-relation-broken" (via hook dispatching script: dispatch) failed: exit status 1

Testing Instructions

Upgrade Notes

[lucabello]

This is great, thanks for solving it :)