What happened?

Description

I don't seem to be able to create a new structure on Craft 5.6.0.1

I get "Trying to access array offset on null":

This caught me off-guard so I confirmed the issue by trying to perform the same action on a site running 5.5.9 and it works fine.

After upgrading that site to 5.6.0.1 I encounter the problem again

Steps to reproduce

1. Create a structure

Expected behavior

I should be able to create a structure

Actual behavior

I get "Trying to access array offset on null"

Workaround

If I create the section as a different type (Channel for example), then go back and update it to a structure I can get around this issue.

Craft CMS version

5.6.0.1

PHP version

8.3

Operating system and version

MacOS

Database type and version

MariaDB

Image driver and version

N/A

Installed plugins and versions

• Base install, no plugins