def get_default_selector_name(self) -> Union[str, None]:
        """This function fetch the default selector to use on `dbt run` (if any)
        :return: either a selector if default is set or None
        :rtype: Union[SelectionSpec, None]
        """
        for selector_name, selector in self.selectors.items():
            if selector["default"] is True:
                return selector_name

        return None
This function fetch the default selector to use on `dbt run` (if any)
        :return: either a selector if default is set or None
        :rtype: Union[SelectionSpec, None]
        
core/dbt/config/project.py