Crash when using latex

[miguel-negrao]

sphinx is crashing when building latex.

# Platform:         linux; (Linux-6.1.0-28-amd64-x86_64-with-glibc2.36)
# Sphinx version:   7.1.2
# Python version:   3.11.2 (CPython)
# Docutils version: 0.20.1
# Jinja2 version:   3.1.4
# Pygments version: 2.18.0

# Last messages:
#   part1/3_Ambisonics
#   part1/4_Ambisonics_Unreal_Engine
#   part1/5_Reverb
#   part1/6_vr
#   part1/7_project_guidelines_1
#   part2/index
#   part2/1_introduction_synthesis
#   
#   resolving references...
#   failed

# Loaded extensions:
#   sphinx.ext.mathjax (7.1.2)
#   alabaster (0.7.13)
#   sphinxcontrib.applehelp (1.0.4)
#   sphinxcontrib.devhelp (1.0.2)
#   sphinxcontrib.htmlhelp (2.0.1)
#   sphinxcontrib.serializinghtml (1.1.5)
#   sphinxcontrib.qthelp (1.0.3)
#   myst_parser (3.0.1)
#   sphinx_markdown_builder (unknown version)
#   sphinxcontrib.bibtex (2.6.3)
#   sphinx_subfigure (unknown version)

# Traceback:
Traceback (most recent call last):
  File "/home/miguel/.cache/pypoetry/virtualenvs/sphynx-website-2024-25-lCh1Vbw3-py3.11/lib/python3.11/site-packages/sphinx/cmd/build.py", line 290, in build_main
    app.build(args.force_all, args.filenames)
  File "/home/miguel/.cache/pypoetry/virtualenvs/sphynx-website-2024-25-lCh1Vbw3-py3.11/lib/python3.11/site-packages/sphinx/application.py", line 351, in build
    self.builder.build_update()
  File "/home/miguel/.cache/pypoetry/virtualenvs/sphynx-website-2024-25-lCh1Vbw3-py3.11/lib/python3.11/site-packages/sphinx/builders/__init__.py", line 287, in build_update
    self.build(['__all__'], to_build)
  File "/home/miguel/.cache/pypoetry/virtualenvs/sphynx-website-2024-25-lCh1Vbw3-py3.11/lib/python3.11/site-packages/sphinx/builders/__init__.py", line 360, in build
    self.write(docnames, list(updated_docnames), method)
  File "/home/miguel/.cache/pypoetry/virtualenvs/sphynx-website-2024-25-lCh1Vbw3-py3.11/lib/python3.11/site-packages/sphinx/builders/latex/__init__.py", line 294, in write
    doctree = self.assemble_doctree(
              ^^^^^^^^^^^^^^^^^^^^^^
  File "/home/miguel/.cache/pypoetry/virtualenvs/sphynx-website-2024-25-lCh1Vbw3-py3.11/lib/python3.11/site-packages/sphinx/builders/latex/__init__.py", line 360, in assemble_doctree
    self.env.resolve_references(largetree, indexfile, self)
  File "/home/miguel/.cache/pypoetry/virtualenvs/sphynx-website-2024-25-lCh1Vbw3-py3.11/lib/python3.11/site-packages/sphinx/environment/__init__.py", line 658, in resolve_references
    self.apply_post_transforms(doctree, fromdocname)
  File "/home/miguel/.cache/pypoetry/virtualenvs/sphynx-website-2024-25-lCh1Vbw3-py3.11/lib/python3.11/site-packages/sphinx/environment/__init__.py", line 670, in apply_post_transforms
    transformer.apply_transforms()
  File "/home/miguel/.cache/pypoetry/virtualenvs/sphynx-website-2024-25-lCh1Vbw3-py3.11/lib/python3.11/site-packages/sphinx/transforms/__init__.py", line 80, in apply_transforms
    super().apply_transforms()
  File "/home/miguel/.cache/pypoetry/virtualenvs/sphynx-website-2024-25-lCh1Vbw3-py3.11/lib/python3.11/site-packages/docutils/transforms/__init__.py", line 182, in apply_transforms
    transform.apply(**kwargs)
  File "/home/miguel/.cache/pypoetry/virtualenvs/sphynx-website-2024-25-lCh1Vbw3-py3.11/lib/python3.11/site-packages/sphinx/transforms/post_transforms/__init__.py", line 37, in apply
    self.run(**kwargs)
  File "/home/miguel/.cache/pypoetry/virtualenvs/sphynx-website-2024-25-lCh1Vbw3-py3.11/lib/python3.11/site-packages/sphinx_subfigure/tr_latex.py", line 81, in run
    flattened[-1][2] = False
    ~~~~~~~~~^^^^
IndexError: list index out of range

The code that causes it is:

````{subfigure} B|A

```{image} ../images/synth2_envelope_scale.png
```
```{image} ../images/synth2_envelope_scale_2.png
```

````

and it is fixed by changing "B|A" to "A|B".

There is no error when building html.