Error in CSQASampler()

[j-i-k-o]

Describe the bug
CSQASampler() causes the NoneType-related error.

To Reproduce

>>> import openjij as oj
>>> h = {0: -1, 1: -1, 2: 1, 3: 1}
>>> J = {(0, 1): -1, (3, 4): -1}
>>> sampler = oj.CSQASampler()
>>> res = sampler.sample_ising(h, J)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.9/site-packages/openjij/sampler/csqa_sampler.py", line 103, in sample_ising
    self._annealing_schedule_setting(
  File "/usr/local/lib/python3.9/site-packages/openjij/sampler/sqa_sampler.py", line 228, in _annealing_schedule_setting
    self._schedule, beta_gamma = quartic_ising_schedule(
  File "/usr/local/lib/python3.9/site-packages/openjij/sampler/sqa_sampler.py", line 270, in quartic_ising_schedule
    s = np.linspace(0, 1, num_sweeps)
  File "<__array_function__ internals>", line 5, in linspace
  File "/usr/local/lib/python3.9/site-packages/numpy/core/function_base.py", line 120, in linspace
    num = operator.index(num)
TypeError: 'NoneType' object cannot be interpreted as an integer

Expected behavior

• No error outputs

Environment (please complete the following information):

• OS: Arch Linux
• Version: Python 3.9.6

Additional context