Custom inspector: Object field null after leaving prefab mode or start game or...

Unity Engine
1 
//Script A\\
public class A : MonoBehaviour {
   public B b;

   [System.Serializable]
   public class B {
      public GameObject go;
   }
}
// Script A's custom editor\\
[CustomEditor(typeof(A))]
public class AEditor : Editor {
   A t;

   private void OnEnable() {
      t = target as A;
   }
 
   public override void OnInspectorGUI() {
      t.b.go = EditorGUILayout.ObjectField("Go", t.b.go, typeof(GameObject), true) as GameObject;
   }
}

Why? I stuck here for awhile and cant find a solution. Im using 2020.1.0a14.

2

It only save object data if i edit prefab in scene and apply change to prefab. Ridiculous