Ceph: TypeError in Ceph Dashboard when accessing /api/osd/settings.

Issue

TypeError in Ceph Dashboard when accessing /api/osd/settings.

With dashboard debug enabled, the following error is seen in the logs when the system accesses the /api/osd/settings endpoint:

Traceback (most recent call last):
  File "/usr/share/ceph/mgr/dashboard/controllers/osd.py", line 173, in settings
    'nearfull_ratio': results['nearfull_ratio'],
TypeError: string indices must be integers

There are many ways to get to the same issue, example:

Jul 17 20:14:04 ceph-node-02 ceph-mgr2127: [dashboard INFO request] [::ffff:172.18.101.1:50930] [GET] [200] [0.072s] [admin] [97.0B] /api/health/get_cluster_capacity
Jul 17 20:14:04 ceph-node-02 ceph-mgr2127: [dashboard ERROR ceph_service] unable to load command response as json
Jul 17 20:14:04 ceph-node-02 ceph-mgr2127: [dashboard ERROR exception] Internal Server Error
Traceback (most recent call last):
File "/usr/share/ceph/mgr/dashboard/services/exception.py", line 47, in dashboard_exception_handler
return handler(args, **kwargs)
File "/lib/python3.6/site-packages/cherrypy/_cpdispatch.py", line 54, in call
return self.callable(*self.args, **self.kwargs)
File "/usr/share/ceph/mgr/dashboard/controllers/_base_controller.py", line 263, in inner
ret = func(*args, **kwargs)
File "/usr/share/ceph/mgr/dashboard/controllers/_rest_controller.py", line 191, in wrapper
return func(*vpath, **params)
File "/usr/share/ceph/mgr/dashboard/controllers/osd.py", line 173, in settings
'nearfull_ratio': result['nearfull_ratio'],
TypeError: string indices must be integers
Jul 17 20:14:04 ceph-node-02 ceph-mgr2127: [dashboard ERROR request] [::ffff:172.18.101.1:50922] [GET] [500] [1.702s] [admin] [513.0B] /api/osd/settings
Jul 17 20:14:04 ceph-node-02 ceph-mgr2127: [dashboard ERROR request] [b'{"status": "500 Internal Server Error", "detail": "The server encountered an unexpected condition which prevented it from fulfilling the request.", "request_id": "b3fb6df8-4bfa-4774-b173-1e0552a3a1d3"}