UnboundLocalError in convert.py

[funkyfuture]

Describe the bug

context
I added myst-nb to an existing documentation that includes a .ipynb file and invoke make html.

expectation
A neat result.

bug
But instead 🐛UnboundLocalError🐛 happens
Here's an error message I ran into...

# Sphinx version: 5.3.0
# Python version: 3.11.2 (CPython)
# Docutils version: 0.19 
# Jinja2 version: 3.1.2
# Last messages:
#   updating environment:
#   [new config]
#   11 added, 0 changed, 0 removed
#   reading sources... [  9%] api
#   reading sources... [ 18%] api-note
#   reading sources... [ 27%] changes
#   reading sources... [ 36%] design
#   reading sources... [ 45%] extending
#   reading sources... [ 54%] genindex
#   reading sources... [ 63%] getting_started
# Loaded extensions:
#   sphinx.ext.mathjax (5.3.0) from /home/user/.local/share/hatch/env/virtual/project/pFhfpXCQ/docs/lib/python3.11/site-packages/sphinx/ext/mathjax.py
#   sphinxcontrib.applehelp (1.0.4) from /home/user/.local/share/hatch/env/virtual/project/pFhfpXCQ/docs/lib/python3.11/site-packages/sphinxcontrib/applehelp/__init__.py
#   sphinxcontrib.devhelp (1.0.2) from /home/user/.local/share/hatch/env/virtual/project/pFhfpXCQ/docs/lib/python3.11/site-packages/sphinxcontrib/devhelp/__init__.py
#   sphinxcontrib.htmlhelp (2.0.1) from /home/user/.local/share/hatch/env/virtual/project/pFhfpXCQ/docs/lib/python3.11/site-packages/sphinxcontrib/htmlhelp/__init__.py
#   sphinxcontrib.serializinghtml (1.1.5) from /home/user/.local/share/hatch/env/virtual/project/pFhfpXCQ/docs/lib/python3.11/site-packages/sphinxcontrib/serializinghtml/__init__.py
#   sphinxcontrib.qthelp (1.0.3) from /home/user/.local/share/hatch/env/virtual/project/pFhfpXCQ/docs/lib/python3.11/site-packages/sphinxcontrib/qthelp/__init__.py
#   alabaster (0.7.13) from /home/user/.local/share/hatch/env/virtual/project/pFhfpXCQ/docs/lib/python3.11/site-packages/alabaster/__init__.py
#   autoclasstoc (1.6.0) from /home/user/.local/share/hatch/env/virtual/project/pFhfpXCQ/docs/lib/python3.11/site-packages/autoclasstoc/__init__.py
#   myst_nb (0.17.1) from /home/user/.local/share/hatch/env/virtual/project/pFhfpXCQ/docs/lib/python3.11/site-packages/myst_nb/__init__.py
#   sphinx.ext.autodoc.preserve_defaults (5.3.0) from /home/user/.local/share/hatch/env/virtual/project/pFhfpXCQ/docs/lib/python3.11/site-packages/sphinx/ext/autodoc/preserve_defaults.py
#   sphinx.ext.autodoc.type_comment (5.3.0) from /home/user/.local/share/hatch/env/virtual/project/pFhfpXCQ/docs/lib/python3.11/site-packages/sphinx/ext/autodoc/type_comment.py
#   sphinx.ext.autodoc.typehints (5.3.0) from /home/user/.local/share/hatch/env/virtual/project/pFhfpXCQ/docs/lib/python3.11/site-packages/sphinx/ext/autodoc/typehints.py
#   sphinx.ext.autodoc (5.3.0) from /home/user/.local/share/hatch/env/virtual/project/pFhfpXCQ/docs/lib/python3.11/site-packages/sphinx/ext/autodoc/__init__.py
#   sphinx.ext.autosummary (5.3.0) from /home/user/.local/share/hatch/env/virtual/project/pFhfpXCQ/docs/lib/python3.11/site-packages/sphinx/ext/autosummary/__init__.py
#   sphinx.ext.doctest (5.3.0) from /home/user/.local/share/hatch/env/virtual/project/pFhfpXCQ/docs/lib/python3.11/site-packages/sphinx/ext/doctest.py
#   sphinx.ext.intersphinx (5.3.0) from /home/user/.local/share/hatch/env/virtual/project/pFhfpXCQ/docs/lib/python3.11/site-packages/sphinx/ext/intersphinx.py
#   sphinx.ext.viewcode (5.3.0) from /home/user/.local/share/hatch/env/virtual/project/pFhfpXCQ/docs/lib/python3.11/site-packages/sphinx/ext/viewcode.py
#   sphinxcontrib.fulltoc (unknown version) from /home/user/.local/share/hatch/env/virtual/project/pFhfpXCQ/docs/lib/python3.11/site-packages/sphinxcontrib/fulltoc.py
Traceback (most recent call last):
  File "/home/user/.local/share/hatch/env/virtual/project/pFhfpXCQ/docs/lib/python3.11/site-packages/sphinx/cmd/build.py", line 281, in build_main
    app.build(args.force_all, args.filenames)
  File "/home/user/.local/share/hatch/env/virtual/project/pFhfpXCQ/docs/lib/python3.11/site-packages/sphinx/application.py", line 347, in build
    self.builder.build_update()
  File "/home/user/.local/share/hatch/env/virtual/project/pFhfpXCQ/docs/lib/python3.11/site-packages/sphinx/builders/__init__.py", line 310, in build_update
    self.build(to_build,
  File "/home/user/.local/share/hatch/env/virtual/project/pFhfpXCQ/docs/lib/python3.11/site-packages/sphinx/builders/__init__.py", line 326, in build
    updated_docnames = set(self.read())
                           ^^^^^^^^^^^
  File "/home/user/.local/share/hatch/env/virtual/project/pFhfpXCQ/docs/lib/python3.11/site-packages/sphinx/builders/__init__.py", line 433, in read
    self._read_serial(docnames)
  File "/home/user/.local/share/hatch/env/virtual/project/pFhfpXCQ/docs/lib/python3.11/site-packages/sphinx/builders/__init__.py", line 454, in _read_serial
    self.read_doc(docname)
  File "/home/user/.local/share/hatch/env/virtual/project/pFhfpXCQ/docs/lib/python3.11/site-packages/sphinx/builders/__init__.py", line 510, in read_doc
    publisher.publish()
  File "/home/user/.local/share/hatch/env/virtual/project/pFhfpXCQ/docs/lib/python3.11/site-packages/docutils/core.py", line 224, in publish
    self.document = self.reader.read(self.source, self.parser,
                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/user/.local/share/hatch/env/virtual/project/pFhfpXCQ/docs/lib/python3.11/site-packages/sphinx/io.py", line 104, in read
    self.parse()
  File "/home/user/.local/share/hatch/env/virtual/project/pFhfpXCQ/docs/lib/python3.11/site-packages/docutils/readers/__init__.py", line 76, in parse
    self.parser.parse(self.input, document)
  File "/home/user/.local/share/hatch/env/virtual/project/pFhfpXCQ/docs/lib/python3.11/site-packages/myst_nb/sphinx_.py", line 87, in parse
    notebook = nb_reader.read(inputstring)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/user/.local/share/hatch/env/virtual/project/pFhfpXCQ/docs/lib/python3.11/site-packages/myst_nb/core/read.py", line 38, in standard_nb_read
    return nbf.reads(text, as_version=NOTEBOOK_VERSION)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/user/.local/share/hatch/env/virtual/project/pFhfpXCQ/docs/lib/python3.11/site-packages/nbformat/__init__.py", line 91, in reads
    nb = convert(nb, as_version)
         ^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/user/.local/share/hatch/env/virtual/project/pFhfpXCQ/docs/lib/python3.11/site-packages/nbformat/converter.py", line 56, in convert
    converted = convert_function(nb)
                ^^^^^^^^^^^^^^^^^^^^
  File "/home/user/.local/share/hatch/env/virtual/project/pFhfpXCQ/docs/lib/python3.11/site-packages/nbformat/v2/convert.py", line 47, in upgrade
    ws.cells.append(newcell)
                    ^^^^^^^
UnboundLocalError: cannot access local variable 'newcell' where it is not associated with a value

Reproduce the bug

see above.

List your environment

No response

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