How to reproduce:

1. Open any project with custom editor windows and open those
2. Save the project and quit
3. Open Unity and create a new project
4. Look at the Console window

Expected behavior: Unity shouldn't display errors when creating a new/empty project because window layout settings from a previous project couldn't be restored in the current one
Actual behavior: Error messages are displayed in the Console window

Reproducible: 5.4.4p1, 5.5.0p4, 5.6.0b3
Not reproducible: 5.3.7p3