Crash with argon_nvt.vm example

$ texts script -f argon_nvt.vm
Traceback (most recent call last):
  File "vre-language/src/virtmat/language/utilities/errors.py", line 302, in wrapper
    return func(*args, **kwargs)
  File "vre-language/src/virtmat/language/cli/run_model.py", line 74, in run_instant_deferred
    return meta.model_from_file(clargs.model_file, deferred_mode=deferred,
  File "jupyter-tensorflow_2023-09-12/lib64/python3.9/site-packages/textx/metamodel.py", line 715, in model_from_file
    return self.internal_model_from_file(
  File "jupyter-tensorflow_2023-09-12/lib64/python3.9/site-packages/textx/metamodel.py", line 781, in internal_model_from_file
    p(model, self)
  File "vre-language/src/virtmat/language/constraints/typechecks.py", line 35, in check_types_processor
    _ = obj.type_
  File "/usr/lib64/python3.9/functools.py", line 993, in __get__
    val = self.func(instance)
  File "vre-language/src/virtmat/language/utilities/errors.py", line 207, in wrapper
    return func(*args, **kwargs)
  File "vre-language/src/virtmat/language/constraints/typechecks.py", line 84, in variable_type
    return self.parameter.type_
  File "/usr/lib64/python3.9/functools.py", line 993, in __get__
    val = self.func(instance)
  File "vre-language/src/virtmat/language/utilities/errors.py", line 207, in wrapper
    return func(*args, **kwargs)
  File "vre-language/src/virtmat/language/constraints/typechecks.py", line 475, in iterable_property_type
    if self.obj.type_ is None:
  File "/usr/lib64/python3.9/functools.py", line 993, in __get__
    val = self.func(instance)
  File "vre-language/src/virtmat/language/utilities/errors.py", line 211, in wrapper
    raise TextXError(str(err), **get_location_context(obj)) from err
textx.exceptions.TextXError: vre-language/examples/argon_nvt.vm:49:8: '<' not supported between instances of 'int' and 'NoneType' => 'cells[0]'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "jupyter-tensorflow_2023-09-12/bin/texts", line 8, in <module>
    sys.exit(texts())
  File "vre-language/src/virtmat/language/cli/__init__.py", line 28, in texts
    clargs.func(clargs)
  File "vre-language/src/virtmat/language/cli/run_model.py", line 149, in main
    prog = run_instant_deferred(clargs, deferred=False)
  File "vre-language/src/virtmat/language/utilities/errors.py", line 304, in wrapper
    process_error(err)
  File "vre-language/src/virtmat/language/utilities/errors.py", line 292, in process_error
    raise err.__cause__
  File "vre-language/src/virtmat/language/utilities/errors.py", line 207, in wrapper
    return func(*args, **kwargs)
  File "vre-language/src/virtmat/language/constraints/typechecks.py", line 467, in general_reference_type
    type_ = get_property_type(self.ref, type_, accessor)
  File "vre-language/src/virtmat/language/constraints/typechecks.py", line 441, in get_property_type
    if not abs(accessor.index) < type_.datalen[0]:
TypeError: '<' not supported between instances of 'int' and 'NoneType'