Editor window set Singleton List

Hi how i can set List in sigleton ?

I have class:

 // Singleton, only one must exist in scene
     private static GuiLayers _instance = null;
     public static GuiLayers instance {
         get{
             // In GuiLayers is not yeat created
             if(!_instance){
                 // Try get JobManger object in scene
                 _instance = FindObjectOfType(typeof(GuiLayers)) as GuiLayers;
                 
                 // If is not is scene
                 if(!_instance){
                     // Create a new one
                     var instance = new GameObject("GuiLayers");
                     _instance = instance.AddComponent<GuiLayers>();
                 }
             }
             
             return _instance;
         }
     }
     
     public int ActiveLayer = -1; // This set working fine
     
     public List<GameObject> GuiGameObjects = new List<GameObject>(); // This collection is always null

In my custom window editor i select the GO with GuiLayers script, and I set the ActiveLayer and GuiGameObjects.

Them if i Play, the ActiveLayer is set but GuiGameObjects is null. Where is the problem ?