Keep mono reference in the asset of the private class

I started playing with custom editors and got an idea to have something like config for the editors. So I've created simple method for that purpose:

 public static T LoadOrCreateConfig<T>(this Editor editor) where T : ScriptableObject
     {
         var path = GetRequiredConfigName<T>(editor);
         var config = AssetDatabase.LoadAssetAtPath<T>(path);
         if (config == null)
         {
             AssetDatabase.DeleteAsset(path);
             config = ScriptableObject.CreateInstance<T>();
             AssetDatabase.CreateAsset(config, path);
         }
         return config;
     }
 
     private static string GetRequiredConfigName<T>(Editor editor)
     {
         var mono = MonoScript.FromScriptableObject(editor);
         var path = AssetDatabase.GetAssetPath(mono);
         return PathUtil.ExcludeFileExtension(path) + "_config.asset";
     }

So predicted common usage may look like:

 public class MyCustomEditor : Editor
 {
     private class MyCustomEditorConfig : ScriptableObject { }
 
     private MyCustomEditorConfig _config;
 
     private void OnEnable()
     {
         _config = this.LoadOrCreateConfig<MyCustomEditorConfig>();
     }
 }

The problem is that, if I declare config class that way (inside editor class) then it breaks each time unity restart, it just says me that reference to the mono script is lost. I want to use private class just in order to be sure that all config is encapsulated and no one can access it (and hide it in the IntelliSense as well). I'm trying to use ScriptableObject instead of JSON, XML, INI etc., because I can create custom drawers for that config and it is easy to setup reference to the complicated objects, those like textures, gameobjects, mono etc. So the question, is there a way to keep reference to the class declared inside the other class?