"ScriptableSingleton already exists. Did you query the singleton in a constructor?" exception is thrown on removing any Package.

Steps to repro:
1. Create a new Project.
2. Windows > Package Manager > 2D Sprite > Install.
3. Select 2D Sprite > Remove Package.

Actual Result:
ScriptableSingleton already exists exception is thrown on removing 2d Sprite Package.

Expected Result:
2D Sprite Package should be removed without any error in the console.

NOTE:
On installing 2D Sprite package "ReleaseAllScriptCaches did not release all script caches!" exception is thrown.

Reproducible in:
Unity Version: 2019.3.0a8 & Package Version: 1.0.0.

Working fine in:
Unity Version: 2019.3.0a7 & Package Version: 1.0.0,
Unity Version: 2019.2.0b8 & Package Version: 1.0.0.

Environment:
Only on Windows.

Stack trace:
ScriptableSingleton already exists. Did you query the singleton in a constructor?
UnityEditor.PackageManager.UI.AssetStore.AssetStoreOAuthInternal:.ctor()