"NullReferenceException:Object reference not set to an instance of an object UnityEditor.DockArea:OnEnable()" throws when editor reopened with sprite editor docked.

Steps to reproduce:
1) Create a new project.
2) Window > Package manager> 2D Sprite > Install
3) Import any sprite in the project.
4) Import settings > texture type > Sprite 2D and UI.
5) Hit Apply > Sprite Editor.
6) Dock Sprite Editor window.
7) Quit Unity.
8) Reopen Unity.

Actual result: Throws NullReference Exception when the editor reopened with sprite editor docked.

Occurring on: 2019.3.0a9, 2019.2.0b9, 2019.2.0b1
Working on: 2019.2.0a14

Environment: Windows and Mac.