gameobject has been destroyed you are still trying to access it

im getting this message and im sure its because when my player gets hit he loses all his gems, the gems flash and then there destroyed, following this message i get an odd message about matrix stack full depth reached but i cant see how theyre related, any how im instantiating a prefab making it flash using materials for a couple seconds and then destroying themso my error is because the flashing is still happening after there destroyed but i am already checking if the gameobject is null can anybody give me a hand? here is half the code to make the gems flash

   void coinFlash()
 {
     newCoins = GameObject.FindGameObjectsWithTag("newcoins");
     if (newCoins != null)
     {
         for (var i = 0; i < newCoins.Length; i++)
         {
             //coinPrefab.GetComponent<Renderer>().sharedMaterial.color = new Color32(250, 250, 210, 0);
             coinPrefab.GetComponent<Renderer>().sharedMaterial = goldtrans;
         }
     }
 }