I can't setting at once ModelImporter.sourceAvatar in script.

I can't setting ModelImporter.sourceAvatar in script at once.

         var avatarAsset = AssetDatabase.LoadAssetAtPath(modelAssetPath, typeof(Avatar)) as Avatar;
         Debug.Log("IsValid : " + avatarAsset.isValid);
         importer.sourceAvatar = avatarAsset;
 

When I run above the code. importer.sourceAvatar is not applied at once. It's only working out as pressing "update' button in Inspector

How I can setting the problem at once? without update button.