FooConfig.cs

using System;
using System.IO;
using System.Linq;
#if UNITY_EDITOR
using UnityEditor;
#endif
using UnityEngine;

public class FooConfig : ScriptableObject
{
    private static FooConfig _instance;
    
    public static FooConfig Instance
    {
        get
        {
#if UNITY_EDITOR
            if (_instance == null)
            {
                var asset = PlayerSettings.GetPreloadedAssets().OfType<FooConfig>().FirstOrDefault();
                _instance = asset != null ? asset : CreateDefault();
            }

            return _instance;

#else
            if (_instance == null)
            {
                _instance = CreateInstance<FooConfig>();
            }

            return _instance;
#endif
        }
        private set => _instance = value;
    }

    private void OnEnable()
    {
        _instance = this;
    }

#if UNITY_EDITOR
    [MenuItem("Assets/Create/FooConfig")]
    private static void Create()
    {
        
        var asset = PlayerSettings.GetPreloadedAssets().OfType<FooConfig>().FirstOrDefault();
        if (asset != null)
        {
            throw new InvalidOperationException($"{nameof(FooConfig)} already exists in preloaded assets");
        }

        var assetPath = EditorUtility.SaveFilePanelInProject($"Save {nameof(FooConfig)}", nameof(FooConfig),
            "asset", "", "Assets");

        if (string.IsNullOrEmpty(assetPath))
        {
            return;
        }

        var folderPath = Path.GetDirectoryName(assetPath);
        if (!string.IsNullOrEmpty(folderPath) && !Directory.Exists(folderPath))
        {
            Directory.CreateDirectory(folderPath);
        }

        var instance = CreateInstance<FooConfig>();
        AssetDatabase.CreateAsset(instance, assetPath);
        var preloadedAssets = PlayerSettings.GetPreloadedAssets().ToList();
        preloadedAssets.Add(instance);
        PlayerSettings.SetPreloadedAssets(preloadedAssets.ToArray());
        AssetDatabase.SaveAssets();
    }
#endif
}