def _validate_repo_providers(self, proposal):
        """trait validator to ensure there is at least one repo provider"""
        if not proposal.value:
            raise TraitError("Please provide at least one repo provider")

        if any(
            [
                not issubclass(provider, RepoProvider)
                for provider in proposal.value.values()
            ]
        ):
            raise TraitError(
                "Repository providers should inherit from 'binderhub.RepoProvider'"
            )

        return proposal.value
trait validator to ensure there is at least one repo provider
binderhub/app.py