NullReferenceException is thrown on deleting Preset Asset.
Steps t repro:
1. Open the attached project.
2. Select Box Collider Preset Asset from Project window.
3. Delete BoxCollider Preset Asset.

Actual result:
NullReferenceException is thrown in the console.

Expected result:
Preset asset deletes from the Project window.

Occurring in:
2019.3.0a3, 2019.3.0a1.

Working Fine in:
2019.2.0b2.

Environment:
Windows & Mac