Returns the default :class:`~horizon.Dashboard` instance.

        If ``"default_dashboard"`` is specified in ``HORIZON_CONFIG``
        then that dashboard will be returned. If not, the first dashboard
        returned by :func:`~horizon.get_dashboards` will be returned.

def get_default_dashboard(self):
        """Returns the default :class:`~horizon.Dashboard` instance.

        If ``"default_dashboard"`` is specified in ``HORIZON_CONFIG``
        then that dashboard will be returned. If not, the first dashboard
        returned by :func:`~horizon.get_dashboards` will be returned.
        """
        if self.default_dashboard:
            return self._registered(self.default_dashboard)
        elif self._registry:
            return self.get_dashboards()[0]
        else:
            raise NotRegistered("No dashboard modules have been registered.")