How to reproduce:

1. Create and open a new project
2. Create a Sphere in the Hierarchy and make a Prefab of it
3. Go to Window → Package Management → Package Manager
4. Install the “Behavior“ package
5. Drag the “Sphere” Prefab over the Hierarchy window
6. Observe the Hierarchy window and the Console

Actual results: Unable to place the prefab in the Hierarchy, and “ArgumentNullException“ errors are spammed while hovering
Expected results: Able to place the prefab in the Hierarchy, and no errors are thrown

Reproducible in: 6000.4.0a5, 6000.5.0a2
Not reproducible in: 6000.0.63f1, 6000.2.14f1, 6000.3.0f1, 6000.4.0a4

Reproduced on: Windows 11 Pro (24H2)
Not reproduced on: No other environment tested

Notes:
- The first part of the error reads “ArgumentException: Object of type 'System.UInt64' cannot be converted to type 'System.Int32'.”
- In 6000.4.0a4 and some lower versions, 1.0.13 of the Behavior package is supported. However, the problem does not reproduce in those Editor versions