Reproduction steps:
1. Create a new project
2. Open Render Pipeline Converter window (Window > Rendering > Render Pipeline Converter)
3. Enable any of the Converters and press “Initialize Converters” button
4. Enter Play mode
5. Observe Console window

Actual result: “The type … is being serialized by [SerializeReference], but is missing the [Serializable] attribute.” warnings are thrown in the Console
Expected result: No warnings or errors are thrown

Reproducible with: 6000.4.0a1
Not reproducible with: 6000.0.58f1, 6000.2.6f1, 6000.3.0a6, 6000.3.0b3
Couldn’t reproduce with: 2021.3.56f1, 2022.3.67f1 (No Render Pipeline Converter window)

Reproduced on: macOS Sequoia 15.4 (M4)

Notes:

• Double clicking on the error makes the "Path does not exist" window pop up
• Full warning:

{code:java}
The type Unity.RenderPipelines.Universal.Editor UnityEditor.Rendering.Universal.BuiltInToURP2DMaterialUpgrader is being serialized by [SerializeReference], but is missing the [Serializable] attribute. To suppress this warning add [Serializable] or use [MakeSerializable].
{code}