NullReferenceException error is thrown on changing layout to saved layout where the Package Manager window is docked.

Steps to repro:
1. Create a new Project.
2. Windows > Package Manager > Dock it.
3. Save the layout as "PackmanLayout".
4. Change layout to any other layout.
5. Change layout to "PackmanLayout".

Actual Result:
NullReferenceException error in the console.

Expected Result:
No error in the console.

Reproducible on:
2020.1.0a10 (deed8847c343).

Working Fine in:
2020.1.0a9, 2019.3.0b8.

Environment:
Windows 10 & Mac 10.14.

Stack trace:
NullReferenceException:Object reference not set to an instance of an object
UnityEditor.DockArea:OnEnable()
UnityEditorInternal.InternalEditorUtility:LoadSerializedFileAndForget(String)
UnityEditor.<>c__DisplayClass34_0:<ReloadWindowLayoutMenu>b__2()