TypeError: 'NoneType' object is not callable in NodeJSEngine.__del__()

[mr-c]

https://github.com/common-workflow-language/cwltool/runs/7649655102?check_suite_focus=true#step:8:54

Exception ignored in: <bound method NodeJSEngine.__del__ of <cwl_utils.sandboxjs.NodeJSEngine object at 0x7f35bacd44a8>>
  Traceback (most recent call last):
    File "/home/runner/work/cwltool/cwltool/.tox/py36-unit/lib/python3.6/site-packages/cwl_utils/sandboxjs.py", line 105, in __del__
  TypeError: 'NoneType' object is not callable
  Exception ignored in: <bound method NodeJSEngine.__del__ of <cwl_utils.sandboxjs.NodeJSEngine object at 0x7fcf4edc3668>>
  Traceback (most recent call last):
    File "/home/runner/work/cwltool/cwltool/.tox/py36-unit/lib/python3.6/site-packages/cwl_utils/sandboxjs.py", line 105, in __del__
  TypeError: 'NoneType' object is not callable