AttributeError: 'DesignSpace' object has no attribute '_cs' about smt HOT 4 CLOSED

import numpy as np
import matplotlib.pyplot as plt

from smt.surrogate_models import KRG
from smt.applications.mixed_integer import MixedIntegerKrigingModel
from smt.utils.design_space import DesignSpace, IntegerVariable

import pickle

xt = np.array([0.0, 2.0, 3.0])
yt = np.array([0.0, 1.5, 0.9])

design_space = DesignSpace(
    [
        IntegerVariable(0, 4),
    ]
)
sm = MixedIntegerKrigingModel(
    surrogate=KRG(design_space=design_space, theta0=[1e-2])
)
sm.set_training_values(xt, yt)
sm.train()

num = 500
x = np.linspace(0.0, 4.0, num)
y = sm.predict_values(x)
# estimated variance
s2 = sm.predict_variances(x)

filename = "kriging.pkl"
with open(filename, "wb") as f:
   pickle.dump(sm, f)

sm2 = None
filename = "kriging.pkl"
with open(filename, "rb") as f:
   sm2 = pickle.load(f)

y_2 = sm2.predict_values(x)
# estimated variance
s2_2 = sm2.predict_variances(x)

np.testing.assert_almost_equal(y, y_2)
np.testing.assert_almost_equal(s2, s2_2)