Flag -a, --async-run does not work without flag -r, --autorun

If the run_model.py is started only with -a flag then the following error occurs:

Traceback (most recent call last):
  File "vre-language/src/virtmat/language/utilities/errors.py", line 96, in wrapper
    return func(*args, **kwargs)
  File "../scripts/run_model.py", line 75, in run_workflow
    session.stop_runner()
  File "vre-language/src/virtmat/language/interpreter/session_manager.py", line 93, in stop_runner
    self.wfe.stop(join=True)
AttributeError: 'NoneType' object has no attribute 'stop'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "../scripts/run_model.py", line 118, in <module>
    main()
  File "../scripts/run_model.py", line 112, in main
    prog = run_workflow(clargs)
  File "vre-language/src/virtmat/language/utilities/errors.py", line 101, in wrapper
    raise RuntimeError('non-handled exception') from err
RuntimeError: non-handled exception

The error does not occur with run_session.py. I guess it sets autorun automatically if async-run is set.