A number of times it has come up with users wanting to store a 64-bit integer and recently some more came in with users wanting to store a reference to "Entity".  Because "Entity" is two 32-bit integers, effectively a 64-bit integer, adding in a 64-bit integer would make referencing an "Entity" easy using a simple extension method for encoding/decoding it.

To this end, adding in a "UInt64"to "PhysicsUserData" is required.

Ref: [https://discussions.unity.com/t/low-level-physics-2d-physicsbody-transformwrite-and-ecs/1692447/3]

Additionally, it's currently impossible to assign a NULL objectValue.