error in editor.open(mesh, 2, 2)

[hustjinghu]

--------------------------------

purpose: phase field for fracture

Tension test 2D

Hirshikesh, Sundarajan Natarajan, Ratna Kumar Annabatutla

IITM, Aug 2017

#-------------------------------------
from dolfin import *
import numpy as np
import matplotlib.pyplot as plt
set_log_active(False)
#==================================

mesh (concident nodes) for the crack

#===================================
nx = 1
ny = 1
num = 1
mesh = Mesh()
editor = MeshEditor()
editor.open(mesh, 2, 2)

When editor.open(mesh, 2, 2) is run, the python gives the following error:

which shows the editor.open argument parameter of CellType::Type or string is not supply.
My dolfin version is 2017.2.0