When entering or leaving play mode or even changing the color scheme in Preferences throws "NullReferenceException" error. When you double click on the error it comes up with a file that's missing an "Editor window" field.

Steps to reproduce:
1. Open attached project
2. Enter play mode
Optional: Go to Preferences -> Color and change around any of the colors for a more severe effect
3. Observe console for error message

Expected outcome: Nothing out of the ordinary happens. Play mode, color scheme changing throws no errors.
Actual outcome: Console throws "NullReferenceException" error each time some change in GUI happens.

The issue might be Layout related, because if you reset layout to default, the Errors stop appearing.

Reproduced on v 5.5.0p2, 5.5.2f1 and 5.5.3f1
Didn't reproduce on v 5.6.0b3, 2017.1.0a4

Couldn't do regression testing, because trying to open the project on an earlier build says the layout cannot be loaded and the default has to be used. (Getting rid of the error in the process)