User can not close down a custom editor window from a previous project.

To reproduce:
- Import attached "TestWindow.unitypackage" and launch "test" scene
- Go to "Test" in top menu bar an select "CurveField Test"
- Dock the "Test Window" somewhere in the editor
- Save the "test" scene
- Create a new project
- Observe that there is a "Failed to load" window
- Right-click on the tab and observe that nothing happens
- Observe alse the following error in Console:

"Recursive Serialization is not supported. You can't dereference a PPtr while loading. (Constructors of C# classes may not load objects either eg. EditorGUIUtility.TextContent should be moved to OnEnable. See stacktrace.)
UnityEditorInternal.InternalEditorUtility:LoadSerializedFileAndForget(String)
UnityEditor.WindowLayout:LoadWindowLayout(String, Boolean)"