How to reproduce:
1. Create a new project
2. Change the Layout in the Editor by moving and docking windows to different places
3. Close and re-open the project
4. Maximize the Game view by pressing on the vertical ellipsis in the top right of the Game view and then pressing Maximize

Expected results: No errors are thrown
Actual results: The "Invalid editor window UnityEditor.FallbackEditorWindow" error is thrown in the Console

Reproducible with: 2019.4.12f1, 2019.4.14f1
Not reproducible with: 2018.4.28f1, 2019.4.11f1, 2020.1.0a1, 2020.1.11f1, 2020.2.0b9, 2021.1.0a3

Notes:
- Entering the Play Mode while Maximize On Play is enabled will throw the error as well
- Resetting the Layout (by choosing any of the Layouts from the drop-down Layout list) will throw the "Failed to destroy editor windows: #1" error in the Console and will stop the other error from appearing when maximizing the Game view