Steps to reproduce:

1. Create a new Unity project using the Universal 3D template
2. Enter Play Mode
3. Observe the Console window

Actual results: "The type UnityEditor.CoreModule UnityEditor.FallbackSelectionItem is being serialized by [SerializeReference], but is missing the [Serializable] attribute." warning is thrown

Expected results: No warnings are thrown

Reproducible with versions: 6000.3.0a5 (36d7e9fd6645)

{}Not reproducible with{}: 6000.3.0a4

Tested on (OS): macOS Silicon Sequoia 15.5

Notes:
*
The warning also is thrown when uninstalling packages from Package Manager