Как получить доступ к UISpite?

Есть код, который создаёт объекты с компонентом Image, в котором я хочу заменить Source Image на один из дефолтных, через код.

Пример:

Код:

using System.Collections;
using System.Collections.Generic;
using UnityEngine.UI;
using UnityEngine;
public class PotionManager : MonoBehaviour {
    Image sourceImage;
    private float timer=25;
    static int COUNT_OF_ORDERS = 0;
    public Transform parent;
    public List<PotionAsset> potionList = new List<PotionAsset>();
    private float posX,posY,posZ;
    // Use this for initialization
    void Start () {
    }
    // Update is called once per frame
    void Update()
    {
        timer += Time.deltaTime;
        if (timer >= 30)
        {
            CreateOrderHolder();
        }
    }
    void CreateOrderHolder()
    {
        posX = 160 * COUNT_OF_ORDERS + 80;
        posY = -70;
        posZ = 0;
        COUNT_OF_ORDERS++;
        GameObject temp = new GameObject("Potion Order " + COUNT_OF_ORDERS, typeof(RectTransform));
        temp.transform.SetParent(parent);
        temp.transform.localPosition = new Vector3(posX, posY, posZ);
        temp.transform.localScale = new Vector3(1.5f, 1.5f, 1.5f);
        temp.GetComponent<RectTransform>().anchorMax = new Vector2(0, 1);
        temp.GetComponent<RectTransform>().anchorMin = new Vector2(0, 1);
        CreatePotionHolder(temp.transform);
        timer = 0;
    }
    void CreatePotionHolder(Transform parent)
    {
        posX = 0;
        posY = 13.5f;
        posZ = 0;
        GameObject temp = new GameObject("PotionHolder", typeof(RectTransform), typeof(Image));
        temp.transform.SetParent(parent);
        temp.transform.localPosition = new Vector3(posX, posY, posZ);
        temp.transform.localScale = new Vector3(1, 1, 1);
        temp.GetComponent<RectTransform>().sizeDelta = new Vector2(100,50);
        sourceImage = temp.GetComponent<Image>();
        sourceImage.sprite = Resources.Load<Sprite>("Resources/unity_builtin_extra/UISprite");
        Debug.Log(sourceImage.sprite);
    }
}

Итого sourceImage.sprite возвращает null. Пробовал добавлять в конце формат файла png,psd. Не помогло.