`to_python` throws `AttributeError` on `TableBlock` when value is None

[Kat]

Issue Summary

Similar to: #4822

After upgrading one of our sites to 6.0, attempting to update some of our pages' indices results in a server error.

  File "/venv/lib/python3.8/site-packages/wagtail/contrib/table_block/blocks.py", line 135, in to_python
    if not value.get("table_header_choice", ""):
AttributeError: 'NoneType' object has no attribute 'get'

This is because, in Wagtail 6.0, the following code has been added:
fe1a306#diff-f058d7a36746222950f678ed6b178205778e4e1e5b85f0299c2a7c3b9d6c8939R129

Steps to Reproduce

1. Create a page with an empty TableBlock in its body
2. Before Wagtail 6.0, this should save and render OK
3. After upgrading to 6.0, editing the same page will throw an error.
4. Looking at the logs, you should see the error above

A workaround is to identify those pages and remove the empty TableBlocks, but this would prove to be a tedious task if there are many page instances.

A possible fix would be to check if the value is not None first:

if value and not value.get("table_header_choice", ""):

[sag​e]

Thanks for the report! I can confirm this issue using the bakerydemo by creating a new Recipe page with an empty table block.

[LB (Ben Johnston)]

Thanks for the bug report and the quick fix @laymonage