None cannot be a node

Raise code

""" 

        On many platforms hashable items also include mutables such as
        NetworkX Graphs, though one should be careful that the hash
        doesn't change on mutables.
        """
        if node_for_adding not in self._succ:
            if node_for_adding is None:
                raise ValueError("None cannot be a node")
            self._succ[node_for_adding] = self.adjlist_inner_dict_factory()
            self._pred[node_for_adding] = self.adjlist_inner_dict_factory()
            attr_dict = self._node[node_for_adding] = self.node_attr_dict_factory()
            attr_dict.update(attr)
        else:  # update attr even if node already exists
            self._node[node_for_adding].update(attr)