Crash when `.. tab-set::` contains a non `.. tab-item::`

[gertrude-salgado-2346]

Describe the bug

bug

Sphinx -- or more precisely sphinx_design -- crashes when a .. tab-set:: directive contains something other than .. tab-item::.

The following example document illustrates this:
(conf.py is generated from sphinx-quickstart and sphinx_design is added to extensions).

Tab Test document
=================

.. tab-set::

   .. tab-item:: A

      A content

   foo  <------------------ This line is the culprit

   .. tab-item:: B

      B content

Results in:

        Traceback (most recent call last):
      File ".../site-packages/sphinx/cmd/build.py", line 432, in build_main
        app.build(args.force_all, args.filenames)
      File ".../site-packages/sphinx/application.py", line 426, in build
        self.builder.build_update()
      File ".../site-packages/sphinx/builders/__init__.py", line 375, in build_update
        self.build(
      File ".../site-packages/sphinx/builders/__init__.py", line 454, in build
        self.write(docnames, updated_docnames, method)
      File ".../site-packages/sphinx/builders/__init__.py", line 735, in write
        self.write_documents(docnames)
      File ".../site-packages/sphinx/builders/__init__.py", line 749, in write_documents
        self._write_serial(sorted_docnames)
      File ".../site-packages/sphinx/builders/__init__.py", line 765, in _write_serial
        doctree = self.env.get_and_resolve_doctree(docname, self)
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
      File ".../site-packages/sphinx/environment/__init__.py", line 701, in get_and_resolve_doctree
        self.apply_post_transforms(doctree, docname)
      File ".../site-packages/sphinx/environment/__init__.py", line 773, in apply_post_transforms
        transformer.apply_transforms()
      File ".../site-packages/sphinx/transforms/__init__.py", line 92, in apply_transforms
        super().apply_transforms()  # type: ignore[misc]
        ^^^^^^^^^^^^^^^^^^^^^^^^^^
      File ".../site-packages/docutils/transforms/__init__.py", line 182, in apply_transforms
        transform.apply(**kwargs)
      File ".../site-packages/sphinx/transforms/post_transforms/__init__.py", line 46, in apply
        self.run(**kwargs)
      File ".../site-packages/sphinx_design/tabs.py", line 244, in run
        tab_label, tab_content = tab_item.children
        ^^^^^^^^^^^^^^^^^^^^^^
    ValueError: not enough values to unpack (expected 2, got 1)

The version used was sphinx-design 0.6.1.

expectation

I expect sphinx-design not to crash but produce a warning/error message similar to an unknown directive/unkown option/malformed list-table, .... and ignoring the .. tab-set:: directive and its contents.

sphinx-design could even fail more gracefully by reporting the error but parse .. tab-set:: as best as possible, i.e. collect all .. tab-item:: directive and ignore the rest. In this scenario the build output should then contain the tabs which sphinx-design was able to identify.

P.S: Please provide an accurate error location when reporting the reStructuredText error, e.g. line 10 in the example document above.

Reproduce the bug

See example above

List your environment

No response