Reproduction steps:

1. Open "DEL_2017.zip" project
2. "Tools" -> "Level Maker"
3. "Piece Editor"
4. Close both of the windows with the "x" close button in the top right of the custom window

Expected Result: Custom windows close
Actual Result: The Editor crashes

Fixed in: 2018.1.0b1

Reproduced with: 2018.1.0a5, 2017.3.0b11, 2017.2.0p3, 2017.2.0f3, 2017.2.0f1, 2017.2.0b11, 2017.2.0b6, 2017.2.0b1, 2017.2.0a4, 2017.2.0a1 (559228b1ba23), 2017.2.0a1 (42b0c0ab4abe)
Did not reproduce on: 2017.1.2p3, 5.6.4p3 (No longer crashes, but results in a lengthy error)

Note:
"InvalidOperationException: Collection was modified; enumeration operation may not execute.
System.Collections.Generic.List`1+Enumerator[UnityEditor.EditorWindow].VerifyState () (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Collections.Generic/List.cs:778)
System.Collections.Generic.List`1+Enumerator[UnityEditor.EditorWindow].MoveNext () (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Collections.Generic/List.cs:784)
UnityEditor.DockArea.OnDestroy () (at /Users/builduser/buildslave/unity/build/Editor/Mono/GUI/DockArea.cs:89)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)"

The above error often times appears on Mac OS with the reproducible versions, but after a few tries the editor crashes

Does not crash if the windows are closed separately