SetParent after Instantiate. NullReferenceException error

Hello guys. I'm making simple 2D clicker game i have really annoying problem.

I'm trying make popup text after Click, but my "GPC(Clone)" creates outside of Canvas and not visible. I'm trying fix that with this code: gpcClone.transform.SetParent(MainCanvas.transform);

But when i make a click in Game mode, Console write: "NullReferenceException: Object reference not set to an instance of an object" And "GPC(Clone)" again creates outside of Canvas and not visible.

Help me, how i can fix that?

My code:

 using UnityEngine;
 using System.Collections;
 using UnityEngine.UI;
 
 public class Click : MonoBehaviour {
 
     public UnityEngine.UI.Text gpc;
     public UnityEngine.UI.Text gemDisplay;
     public UnityEngine.UI.Text asteroidHealthBar;
     public float gem = 0.00f;
     public int gemperclick = 1;
     public int asteroidFullHealth = 100;
     public GameObject MainCanvas;  // I'm drop in this Game Object my Canvas
 
     void Update() {
         gemDisplay.text = "" + gem.ToString ("F0");
         gpc.text = "+" + gemperclick;
         asteroidHealthBar.text = asteroidFullHealth + " / 100";
     }
 
     public void Clicked() {
         gem += gemperclick;
         asteroidFullHealth -= gemperclick;
         float x = Random.Range(-150, 150);
         float y = Random.Range(100, 200);
         float z = 0;
         gpc.transform.localPosition = new Vector3 (x, y, z);
         GameObject gpcClone = Instantiate (gpc, gpc.transform.localPosition, Quaternion.identity) as GameObject;
         gpcClone.transform.SetParent(MainCanvas.transform);
     }
 
 }