Fix NoneType object is not subscriptable from _insert_filtered_annotations

[khink]

There are cases where the IndirectObject is not None, but d[0] will fail with TypeError: 'NoneType' object is not subscriptable in generic/_base.py, at return self._get_object_with_check()[key].

I previously reported this issue in #3211, but the fix for that issue didn't fix this one.

Unfortunately i still can't attach the PDF file that triggered this issue, because it contains personal information.
If someone has an idea how to create a PDF to test this with, i'd be happy to try.
I fully understand that a maintainer would not be keen on including fixes for rare bugs without a test.

More details on the error

The script i used to reproduce the error:

import glob

from pypdf import PdfWriter


def merge_pdfs():
    with PdfWriter() as merger:
        for pdf in glob.glob("*.pdf"):
            merger.append(pdf)
        merger.write("merged.pdf")

if __name__ == '__main__':

    merge_pdfs()

In the script directory i have a broken.pdf (the offending file i can't share here) and an empty.pdf.

This fails as below:

Object 6 0 not defined.
Object 6 0 not defined.
Overwriting cache for 0 6
Traceback (most recent call last):
  File "/home/kees/Projects/pypdf/check_merge.py", line 21, in <module>
    merge_pdfs()
    ~~~~~~~~~~^^
  File "/home/kees/Projects/pypdf/check_merge.py", line 16, in merge_pdfs
    merger.append(pdf)
    ~~~~~~~~~~~~~^^^^^
  File "/home/kees/Projects/pypdf/pypdf/_writer.py", line 2693, in append
    self.merge(
    ~~~~~~~~~~^
        None,
        ^^^^^
    ...<4 lines>...
        excluded_fields,
        ^^^^^^^^^^^^^^^^
    )
    ^
  File "/home/kees/Projects/pypdf/pypdf/_writer.py", line 2851, in merge
    lst = self._insert_filtered_annotations(
        pag.original_page.get("/Annots", []), pag, srcpages, reader
    )
  File "/home/kees/Projects/pypdf/pypdf/_writer.py", line 3055, in _insert_filtered_annotations
    p = self._get_cloned_page(d[0], pages, reader)
                              ~^^^
  File "/home/kees/Projects/pypdf/pypdf/generic/_base.py", line 402, in __getitem__
    return self._get_object_with_check()[key]  # type: ignore
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^
TypeError: 'NoneType' object is not subscriptable

Setting a breakpoint before the error occurs gives:

(Pdb) d
IndirectObject(6, 0, 127177489623296)
(Pdb) d[0]
Object 6 0 not defined.
Overwriting cache for 0 6
*** TypeError: 'NoneType' object is not subscriptable

(To be fair, in this last example i cheated a bit because the variable name d clashes with the pdb command. So i renamed that.)

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.95%. Comparing base (bc318d7) to head (9a27b3f).

Files with missing lines	Patch %	Lines
pypdf/_writer.py	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3444      +/-   ##
==========================================
- Coverage   96.97%   96.95%   -0.03%     
==========================================
  Files          54       54              
  Lines        9337     9340       +3     
  Branches     1711     1711              
==========================================
+ Hits         9055     9056       +1     
- Misses        168      170       +2     
  Partials      114      114              

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[stefan6419846]

You basically want to check for not is_null_or_none(d[0].get_object())? Having this explicitly is preferred over more costly exception handling.

[stefan6419846]

Thanks for the PR. Please have a look at the failing checks.