TypeError: can only join an iterable

[certik]

We got this curious error message at our CI:

https://gitlab.com/certik/lfortran/-/jobs/1158527677#L460

I could reproduce it locally also:

(xeuslf) lfortran/lfortran(ast1) $ jupyter nbconvert --to notebook --execute --ExecutePreprocessor.timeout=60 --output Demo1_out.ipynb share/lfortran/nb/Demo1.ipynb
[NbConvertApp] Converting notebook share/lfortran/nb/Demo1.ipynb to notebook
Run with XEUS 1.0.1
[NbConvertApp] Executing notebook with kernel: fortran
None
Traceback (most recent call last):
  File "/Users/certik/miniforge3/envs/xeuslf/bin/jupyter-nbconvert", line 11, in <module>
    sys.exit(main())
  File "/Users/certik/miniforge3/envs/xeuslf/lib/python3.9/site-packages/jupyter_core/application.py", line 254, in launch_instance
    return super(JupyterApp, cls).launch_instance(argv=argv, **kwargs)
  File "/Users/certik/miniforge3/envs/xeuslf/lib/python3.9/site-packages/traitlets/config/application.py", line 845, in launch_instance
    app.start()
  File "/Users/certik/miniforge3/envs/xeuslf/lib/python3.9/site-packages/nbconvert/nbconvertapp.py", line 350, in start
    self.convert_notebooks()
  File "/Users/certik/miniforge3/envs/xeuslf/lib/python3.9/site-packages/nbconvert/nbconvertapp.py", line 524, in convert_notebooks
    self.convert_single_notebook(notebook_filename)
  File "/Users/certik/miniforge3/envs/xeuslf/lib/python3.9/site-packages/nbconvert/nbconvertapp.py", line 489, in convert_single_notebook
    output, resources = self.export_single_notebook(notebook_filename, resources, input_buffer=input_buffer)
  File "/Users/certik/miniforge3/envs/xeuslf/lib/python3.9/site-packages/nbconvert/nbconvertapp.py", line 418, in export_single_notebook
    output, resources = self.exporter.from_filename(notebook_filename, resources=resources)
  File "/Users/certik/miniforge3/envs/xeuslf/lib/python3.9/site-packages/nbconvert/exporters/exporter.py", line 181, in from_filename
    return self.from_file(f, resources=resources, **kw)
  File "/Users/certik/miniforge3/envs/xeuslf/lib/python3.9/site-packages/nbconvert/exporters/exporter.py", line 199, in from_file
    return self.from_notebook_node(nbformat.read(file_stream, as_version=4), resources=resources, **kw)
  File "/Users/certik/miniforge3/envs/xeuslf/lib/python3.9/site-packages/nbconvert/exporters/notebook.py", line 32, in from_notebook_node
    nb_copy, resources = super().from_notebook_node(nb, resources, **kw)
  File "/Users/certik/miniforge3/envs/xeuslf/lib/python3.9/site-packages/nbconvert/exporters/exporter.py", line 143, in from_notebook_node
    nb_copy, resources = self._preprocess(nb_copy, resources)
  File "/Users/certik/miniforge3/envs/xeuslf/lib/python3.9/site-packages/nbconvert/exporters/exporter.py", line 318, in _preprocess
    nbc, resc = preprocessor(nbc, resc)
  File "/Users/certik/miniforge3/envs/xeuslf/lib/python3.9/site-packages/nbconvert/preprocessors/base.py", line 47, in __call__
    return self.preprocess(nb, resources)
  File "/Users/certik/miniforge3/envs/xeuslf/lib/python3.9/site-packages/nbconvert/preprocessors/execute.py", line 79, in preprocess
    self.execute()
  File "/Users/certik/miniforge3/envs/xeuslf/lib/python3.9/site-packages/nbclient/util.py", line 74, in wrapped
    return just_run(coro(*args, **kwargs))
  File "/Users/certik/miniforge3/envs/xeuslf/lib/python3.9/site-packages/nbclient/util.py", line 53, in just_run
    return loop.run_until_complete(coro)
  File "/Users/certik/miniforge3/envs/xeuslf/lib/python3.9/asyncio/base_events.py", line 642, in run_until_complete
    return future.result()
  File "/Users/certik/miniforge3/envs/xeuslf/lib/python3.9/site-packages/nbclient/client.py", line 553, in async_execute
    await self.async_execute_cell(
  File "/Users/certik/miniforge3/envs/xeuslf/lib/python3.9/site-packages/nbconvert/preprocessors/execute.py", line 123, in async_execute_cell
    cell, resources = self.preprocess_cell(cell, self.resources, cell_index)
  File "/Users/certik/miniforge3/envs/xeuslf/lib/python3.9/site-packages/nbconvert/preprocessors/execute.py", line 146, in preprocess_cell
    cell = run_sync(NotebookClient.async_execute_cell)(self, cell, index, store_history=self.store_history)
  File "/Users/certik/miniforge3/envs/xeuslf/lib/python3.9/site-packages/nbclient/util.py", line 74, in wrapped
    return just_run(coro(*args, **kwargs))
  File "/Users/certik/miniforge3/envs/xeuslf/lib/python3.9/site-packages/nbclient/util.py", line 53, in just_run
    return loop.run_until_complete(coro)
  File "/Users/certik/miniforge3/envs/xeuslf/lib/python3.9/site-packages/nest_asyncio.py", line 70, in run_until_complete
    return f.result()
  File "/Users/certik/miniforge3/envs/xeuslf/lib/python3.9/asyncio/futures.py", line 201, in result
    raise self._exception
  File "/Users/certik/miniforge3/envs/xeuslf/lib/python3.9/asyncio/tasks.py", line 256, in __step
    result = coro.send(None)
  File "/Users/certik/miniforge3/envs/xeuslf/lib/python3.9/site-packages/nbclient/client.py", line 857, in async_execute_cell
    self._check_raise_for_error(cell, exec_reply)
  File "/Users/certik/miniforge3/envs/xeuslf/lib/python3.9/site-packages/nbclient/client.py", line 760, in _check_raise_for_error
    raise CellExecutionError.from_cell_and_msg(cell, exec_reply_content)
  File "/Users/certik/miniforge3/envs/xeuslf/lib/python3.9/site-packages/nbclient/exceptions.py", line 92, in from_cell_and_msg
    tb = '\n'.join(msg.get('traceback', []))
TypeError: can only join an iterable

and I have a fix coming.