How to reproduce:

1. Open the attached project and notice two scenes are open.
2. Clear the console and close "Windowm8" window if it's opened.
3. Open "Windowm8" window again (Window -> Scene Management) and notice three console logs with the scene count of two.
4. Save the project and exit.
5. Open the project again and notice that three messages in the console now saying only one scene is open.

Expected behavior: Scene management always initialized when OnEnable is called.
Actual behavior: Scene management is not initialized when OnEnable is called for Editor windows on starting Unity.

Reproducible: 5.3.5f1, 5.3.6p3, 5.4.0p2, 5.5.0a6