1. Open a fresh project
2. Create a preset out of anything (components, importers, assets)
3. Select the preset from the project view and delete it

Result: A NullReference Exception error thrown in the console about DestroyInternalEditor()
NullReferenceException: Object reference not set to an instance of an object
UnityEditor.Presets.PresetEditor.DestroyInternalEditor () (at C:/buildslave/unity/build/Modules/PresetsEditor/PresetEditor.cs:174)
UnityEditor.Presets.PresetEditor.OnDisable () (at C:/buildslave/unity/build/Modules/PresetsEditor/PresetEditor.cs:69)

Only reproducible on single selection ... having multiselection, the inspector isn't shown for one single preset so the problem doesn't appear

Regression introduced in 2019.3 ... apparently caused by AssetDatabaseV2 changes.