I did notice that if I assign scripts that generate singleton classes (for my managers), to an empty game object, everything works fine.

Now I would like to start the scene with the empty game object, attach the main game manager singleton, and in the game manager singleton I would like to attach to it the other manager singleton classes.

Although I am not sure how you actually do so. Do you run addComponent to the main game manager, and they will be attached to the existing gameobhect? I need this gameobject to persist, since it get created in the title screen scene, and has to persist as long as the game is running, so make sense to have one game object that hold all the singleton.

I did look for examples on the subject, but the only thing that I found is the reference to add a script to a game object; and I am not sure if that's actually what is supposed to be done.