Trying to find boolean value in another script

I'm trying to check the boolean value from another script, I've read to do this you use the get component however I'm having some trouble with this. In this script I'm trying so that if the user has collected all the items and reaches the goal the player wins. It tells me goalCheckObject hasn't been assigned a variable. Any help is appreciated.

 public class Goal : MonoBehaviour {
     public GameObject goalCheckObject;
     PlayerController goalCheck;
     // Use this for initialization
     void Start () {
         goalCheck = goalCheckObject.GetComponent<PlayerController>();
     }
     
     // Update is called once per frame
     void Update () {
     
     }
 
     void OnTriggerEnter(Collider other){
         if (other.gameObject.name == "Marble" && goalCheck.endGoal == true ){
             Debug.Log(other.gameObject.name + " entered the trigger.");
         }
 
     }
 }