When dragging materials from the project window over the scene view i get the following error.

Undo object may not be null.
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)

Steps to reproduce.
- Open the attached project in Unity 2020.1.0a20
- Select "Assets/8K Skybox Pack Free/Skyboxes/Material/sky-2.mat"
- Drag it into the scene view.
Notice issue >> Undo object may not be null is logged in the console.

Broken with, 2020.1.0a20, 2020.1.0a19, 2020.1.0a18, 2020.1.0a15, 2020.1.0a8
Working with, 2020.1.0a1, 2020.1.0a3, 2020.1.0a4