Steps to reproduce:
1. Open users attached project with Unity 5.4.0b24.
2. Select "Preview" tab in editor.
2. Close "Preview" tab in editor window
--- All tabs will close and you will get "ArgumentOutOfRangeException: Argument is out of range.
Parameter name: index" error. "
--- Note that you will get this error only if you close selected tab.

Reproduced with: 5.4.0b25, 5.4.0b24.
Not reproduced with: 5.3.5f1, 5.4.0b20, 5.4.0b21.
Regression introduced in: 5.4.0b22