Reference to `a:custGeom` descendant or |None| if not present.

def add_path(self, w, h):
        """Reference to `a:custGeom` descendant or |None| if not present."""
        custGeom = self.spPr.custGeom
        if custGeom is None:
            raise ValueError('shape must be freeform')
        pathLst = custGeom.get_or_add_pathLst()
        return pathLst.add_path(w=w, h=h)