can't instantiate gameobject

I have made a button to spawn an object in unity, the only problem is it doesn't spawn that object, i have the script referencing an array from another script which it does actually have a gameobject stored in, but for some reason it is still giving me a null reference error,

below is my code

using UnityEngine; using System.Collections; using UnityEngine.UI;

public class Mspawn : MonoBehaviour { public Button Mbutton; private BoardManager boardManger; // Use this for initialization void Start () { boardManger = GetComponent ();

 }
 
 // Update is called once per frame
 void Update () {
 
 }
 void OnEnable(){
     Mbutton.onClick.AddListener (SpawnM);
 }
 void SpawnM()
 {

     Instantiate (boardManger.PlayerUnits[3], new Vector3 (1, 1, 0f), Quaternion.identity);
 }

}