Failed to destroy editor windows error is thrown on changing Graphics API's.

Steps to repro:
1. Create a new project.
2. File > Build Settings > Player Settings > Other Settings.
3. Uncheck "Auto Graphics APIs for Windows" checkbox.
4. Add Direct3D12 > Drag to the Top of the list > Restart.

Actual Result:
Failed to destroy editor windows error in the console.

Expected Result:
No error in the console.

Reproducible in:
2021.1.0a7.

Working Fine in:
2021.1.0a6, 2020.2.0b12.

Environment:
Occurring on Windows 10 & mac 10.15.

Stack trace:
Failed to destroy editor windows: #1
FallbackEditorWindow
UnityEditor.WindowLayout:LoadDefaultWindowPreferences ()