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Steps to reproduce:
1. Create some RBD images
2. Move (the last) image to trash, sometimes there is an error.
NOTE: the `Move to Trash` operation for the image succeeds.

Screenshot:

See full GIF here: https://imgur.com/a/DqEjiQa

Traceback in backend:

Traceback (most recent call last):
  File "/ceph/src/pybind/mgr/dashboard/services/exception.py", line 100, in handle_rbd_error
    yield
  File "/usr/lib64/python3.7/contextlib.py", line 74, in inner
    return func(*args, **kwds)
  File "/usr/lib64/python3.7/contextlib.py", line 74, in inner
    return func(*args, **kwds)
  File "/ceph/src/pybind/mgr/dashboard/controllers/rbd.py", line 99, in list
    return self._rbd_list(pool_name)
  File "/ceph/src/pybind/mgr/dashboard/controllers/rbd.py", line 92, in _rbd_list
    pool, image['namespace'], image['name']).list()
  File "/ceph/src/pybind/mgr/dashboard/services/rbd.py", line 113, in list
    return _list(ioctx)
  File "/ceph/src/pybind/mgr/dashboard/services/rbd.py", line 101, in _list
    with rbd.Image(ioctx, self._image_name) as image:
  File "rbd.pyx", line 3193, in rbd.Image.__init__
rbd.ImageNotFound: [errno 2] RBD image not found (error opening image b'd' at snapshot None)

A possible cause might be that frontend keeps polling image list and the trashed image is removed during image iteration calls in backend:
https://github.com/ceph/ceph/blob/4c3127d28279510a5117bd5ab11374ad3f5559d4/src/pybind/mgr/dashboard/controllers/rbd.py#L89-L92