Reproduction steps:
1. Open the attached “Repro“ project
2. Select the “Assets/Settings/Renderer2D.asset“ Rederer 2D Data Asset in the Project window
3. Change the Default Material Type to “Custom“ inside the Inspector
4. Observe

Expected result: No errors in the console, Default Material Type changes to Custom
Actual result: NullReferenceException is thrown in the Console, Default Material Type doesn’t change

Reproducible with: 2023.3.0a17, 6000.0.8f1
Not reproducible with: 2022.3.35f1, 2023.3.0a16
Couldn’t test with: 2021.3.40f1 (unable to downgrade)

Reproducible on: M1 Max MacOS 14.5
Not reproducible on: No other environment tested