-ArgumentNullException error is thrown on deleting Tilemap prefab after unpacking the instance of that prefab.
-After this, the error will be thrown for any prefab deletion, if the Tile Palette window is opened.

Steps to repro:
1. Open the attached project.
2. Windows > 2D > Tile Palette.
3. Hierarchy > Select Tilemap > Right Click > Prefab > Unpack Completely..
4. Project Window > Delete Tilemap prefab.

Actual Result:
ArgumentNullException error in thrown the console.

Expected Result:
No error in the console.

Reproducible in:
2020.2.0a9, 2020.2.0a5.

Working Fine in:
2020.2.0a4, 2020.1.0b8.

Environment:
Occurring on Windows 10 & Mac 10.15.

Stack trace:
ArgumentNullException: Value cannot be null.
Parameter name: componentOrGameObject
UnityEditor.PrefabUtility.GetPrefabAssetType (UnityEngine.Object componentOrGameObject) (at <5951c2421c324f47883bd5ba8a4bf507>:0)
UnityEditor.GameObjectInspector.CalculatePrefabStatus () (at <5951c2421c324f47883bd5ba8a4bf507>:0)
UnityEditor.GameObjectInspector.OnForceReloadInspector () (at <5951c2421c324f47883bd5ba8a4bf507>:0)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr, Boolean&)