Interpreter crash with a specific model extension

Let us have an arbitrary persistent mode. Let us extend it with

a = 1; q = b; b = a

The interpreter crashes with:

Traceback (most recent call last):
  File "vre-language/src/virtmat/language/utilities/errors.py", line 211, in wrapper
    return func(*args, **kwargs)
  File "vre-language/src/virtmat/language/cli/run_model.py", line 77, in run_workflow
    session = Session(get_lp(clargs), uuid=clargs.uuid, grammar_path=clargs.grammar_path,
  File "vre-language/src/virtmat/language/interpreter/session_manager.py", line 68, in __init__
    self.get_model(model_str, model_path)
  File "vre-language/src/virtmat/language/interpreter/session_manager.py", line 160, in get_model
    self.model = tx_get_model(model_src, deferred_mode=True,
  File "jupyter-tensorflow-2023-01-02/lib64/python3.8/site-packages/textx/metamodel.py", line 644, in model_from_str
    p(model, self)
  File "vre-language/src/virtmat/language/interpreter/workflow_executor.py", line 400, in workflow_model_processor
    append_nodes(model, new_nodes)
  File "vre-language/src/virtmat/language/interpreter/workflow_executor.py", line 374, in append_nodes
    append_nodes(model, nodes)
  File "vre-language/src/virtmat/language/interpreter/workflow_executor.py", line 376, in append_nodes
    assert all(p is not None for p in parents)
AssertionError

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

Traceback (most recent call last):
  File "jupyter-tensorflow-2023-01-02/bin/texts", line 8, in <module>
    sys.exit(texts())
  File "vre-language/src/virtmat/language/cli/__init__.py", line 25, in texts
    clargs.func(clargs)
  File "vre-language/src/virtmat/language/cli/run_model.py", line 123, in main
    prog = run_workflow(clargs)
  File "vre-language/src/virtmat/language/utilities/errors.py", line 219, in wrapper
    raise RuntimeError('non-handled exception') from err
RuntimeError: non-handled exception