Cannot run example 'examples/calibration_CDM_beta.yaml'

When running the example 'calibration_CDM_beta.yaml', the following error message appears:

$ conda activate pyxel-dev
(pyxel-dev) $ cd pyxel/examples
(pyxel-dev) examples $ python ../pyxel/run.py -c calibration_CDM_beta.yaml
Traceback (most recent call last):
  File "../pyxel/run.py", line 295, in <module>
    main()
  File "../pyxel/run.py", line 289, in main
    run(input_filename=opts.config, random_seed=opts.seed)
  File "../pyxel/run.py", line 183, in run
    simulation.calibration.post_processing(calib_results=results, output=out)
  File "XX/pyxel/pyxel/calibration/calibration.py", line 770, in post_processing
    output.calibration_outputs(processor_list=proc_list)
  File "XX/pyxel/pyxel/util/outputs.py", line 701, in calibration_outputs
    self.single_output(processor)
  File "XX/pyxel/pyxel/util/outputs.py", line 526, in single_output
    assert self.single_plot is not None
AssertionError