RefreshItemAssetPath.cs

[InitializeOnLoadMethod]
public static void RefreshItemAssetPath() {

    EditorApplication.projectWindowChanged += () => {
        foreach (string guid in AssetDatabase.FindAssets("t:Item")) {
            string path = AssetDatabase.GUIDToAssetPath(guid);
            Item item = AssetDatabase.LoadAssetAtPath<Item>(path);
            Match match = new Regex(@"Assets\/Resources\/(?<resource_path>.*)\.asset").Match(path);
            if (!match.Success) {
                Debug.Log("Cannot extract resource Path. : " + path);
                return;
            }
            string resourcePath = match.Groups["resource_path"].Value;
            item.assetPath = resourcePath;
        }
        AssetDatabase.SaveAssets();
    };
}