ng.cs

using UnityEngine;

public class ng : MonoBehaviour
{
  public GameObject prefab;
  private GameObject go;
  private bool Flag = false;

  void Start ()
  {

  }

  void Update ()
  {
    if ((false == Flag)) {
      go = (GameObject)Instantiate(prefab);
      go.transform.parent = transform;
      // 実際にはその場面に応じて変わる
      var animeNo = 0;
      go.GetComponentInChildren<Script_SpriteStudio_PartsRoot>().AnimationPlay(animeNo);
      Flag = true;
    }
  }
}

ok.cs

using UnityEngine;

public class ok : MonoBehaviour
{
  public GameObject prefab;
  private GameObject go;
  private bool Flag = false;

  void Start ()
  {
      go = (GameObject)Instantiate(prefab);
      go.transform.parent = transform;
  }

  void Update ()
  {
    if ((false == Flag)) {
      // 実際にはその場面に応じて変わる
      var animeNo = 0;
      go.GetComponentInChildren<Script_SpriteStudio_PartsRoot>().AnimationPlay(animeNo);
      Flag = true;
    }
  }
}

NullReferenceException: Object reference not set to an instance of an object
Script_SpriteStudio_PartsRoot.AnimationPlay (Int32 No, Int32 TimesPlay, Int32 FrameInitial, Single RateTime, PlayStyle KindStylePlay, System.String LabelStart, Int32 FrameOffsetStart, System.String LabelEnd, Int32 FrameOffsetEnd) (at Assets/SpriteStudio/Script/Script_SpriteStudio_PartsRoot.cs:983)
ng.Update () (at Assets/ng.cs:21)