BlockPreview causes columnSpan: null for nested items in multi-column Block Grid layouts (Umbraco 16.5)

[nwahlberg]

Which Umbraco.Community.BlockPreview version are you using?

No response

Which Umbraco version are you using? For example: 10.4.0 - don't just write v10

16.5

Bug summary

When using BlockPreview (v4.2.2) with Umbraco 16.5, nested Block Grid items inside multi-area layouts (e.g., 9–3 columns) are serialized with "columnSpan": null.

This causes rendering to fail with:

System.InvalidOperationException: Nullable object must have a value.

Notes:

• Full-width layout works correctly
• Issue occurs on brand new content
• Disabling BlockPreview resolves the issue completely

Steps to reproduce

1. Create Block Grid with 12 columns
2. Add layout with two areas (e.g. 9–3)
3. Add any block into each area
4. Inspect stored JSON

Expected result / actual result

Actual result:
"columnSpan": null

Expected result:
"columnSpan": 12