Missing root object raising: 'NoneType' object has no attribute 'get_object' (different from #1295 & #1689)

[BertrandBordage]

As I was processing client PDFs with pypdf, one of them triggered a cryptic error (traceback below).
Ideally, pypdf should raise a PdfReadError (or another subclass of PyPdfError) if that file is really impossible to parse.

Environment

$ python -m platform
Linux-5.15.0-118-generic-x86_64-with-glibc2.35

$ python --version
Python 3.11.9

$ python -c "import pypdf;print(pypdf._debug_versions)"
pypdf==4.3.1, crypt_provider=('local_crypt_fallback', '0.0.0'), PIL=9.5.0

Code

This is a minimal code example that shows the issue:

from pypdf import PdfReader
reader = PdfReader('client file (might be broken).pdf')
list(reader.pages)

I cannot share the PDF as it might contain sensitive client data.

Two messages/warnings are displayed before the traceback, though:

Traceback

This is the complete traceback I see:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/lib/python3.11/site-packages/pypdf/_page.py", line 2227, in __len__
    return self.length_function()
           ^^^^^^^^^^^^^^^^^^^^^^
  File "/lib/python3.11/site-packages/pypdf/_doc_common.py", line 353, in get_num_pages
    self._flatten()
  File "/lib/python3.11/site-packages/pypdf/_doc_common.py", line 1101, in _flatten
    catalog = self.root_object
              ^^^^^^^^^^^^^^^^
  File "/lib/python3.11/site-packages/pypdf/_reader.py", line 191, in root_object
    return cast(DictionaryObject, self.trailer[TK.ROOT].get_object())
                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'get_object'

Additional info

The PDF might be corrupted, as I am unable to open it with Evince, which shows this error: Failed to read the document catalog.
xpdf is also showing various errors when reading the file:

Syntax Error: Couldn't find trailer dictionary
Syntax Error: Invalid XRef entry 493
Internal Error: xref num 493 not found but needed, try to reconstruct<0a>
Syntax Error: Invalid XRef entry 493
Syntax Error: Couldn't find trailer dictionary
Syntax Error: Catalog object is wrong type (null)
Syntax Error: Couldn't find trailer dictionary
Syntax Error: Invalid XRef entry 493
Internal Error: xref num 493 not found but needed, try to reconstruct<0a>
Syntax Error: Invalid XRef entry 493
Syntax Error: Couldn't find trailer dictionary
Syntax Error: Catalog object is wrong type (null)
Syntax Error: Couldn't read page catalog

[stefan6419846]

Thanks for the report. Judging from the stacktrace and the third-party logs, this PDF file just appears to be broken, as apparently the (essential) root object cannot be found.

Feel free to open a PR to convert this into an appropriate exception.