'Test that None as a node in the graph raises an error.'

def test_tree_all_pairs_lowest_common_ancestor11(self):

    G = nx.DiGraph([(None, 3)])
    assert_raises(nx.NetworkXError, list, tree_all_pairs_lca(G))
    assert_raises(nx.NodeNotFound, list, tree_all_pairs_lca(self.DG, pairs=G.edges()))
