message: [dashboard] fix missing datasource issue
  Addresses a bug that happens when rendering a dashboard for which
  a datasource behind one of the slices has been deleted
diff: @@ -1629,7 +1629,12 @@ class Superset(BaseSupersetView):
  qry = qry.filter_by(slug=dashboard_id)
  dash = qry.one()
  - datasources = {slc.datasource for slc in dash.slices}
  + datasources = set()
  + for slc in dash.slices:
  + datasource = slc.datasource
  + if datasource:
  + datasources.add(datasource)
  +
  for datasource in datasources:
  if datasource and not self.datasource_access(datasource):
  flash(
  