Steps to reproduce :

- import the Full Inspector package https://www.assetstore.unity3d.com/en/#!/content/14913
- go to Window > Full Inspector > Developer > Editor Images Manager
- delete the package from the Project Browser
- try to close the empty window and notice the error

*Note :
Right click on the tab doesn't work, needs to be closed with the little 3 lines menu.
This used to work before.

NullReferenceException: Object reference not set to an instance of an object
UnityEditor.EditorWindow.Close () (at /Users/builduser/buildslave/unity/build/Editor/Mono/EditorWindow.cs:679)
UnityEditor.DockArea.Close (System.Object userData) (at /Users/builduser/buildslave/unity/build/Editor/Mono/GUI/DockArea.cs:376)
UnityEditor.GenericMenu.CatchMenu (System.Object userData, System.String[] options, Int32 selected) (at /Users/builduser/buildslave/unity/build/artifacts/generated/common/editor/GenericMenuBindings.gen.cs:122)