Editor -> Popupwindow -> OnLostFocus -> Close() -> Get Windowlayouts error, wait what?

I hope the title describes the problem well enough to get people to look at it. I'm extending the editor, and I made an edit button next to a popup list, that edits the content of said popup list. The mechanic works fine, but that's not the problem.

 public class SomeWindow : EditorWindow {
     //....
     public void OnLostFocus () {
         Close ();
     }         
 }

So, as you can see, I'm trying to close the window when it loses focus. Now, the window actually DOES close, but I get this error:

WindowLayouts are invalid. Please use 'Window -> Layouts -> Revert Factory Settings...' menu to fix it. UnityEditor.HostView:OnLostFocus()

I am completely lost as to what causes this. The error itself doesn't seem to do anything but I don't want it to appear every time.