Object reference not set to an instance of...

Basically, all i am wanting to do is pass through a value from one script (BarScript) to another (EndScreen), this value being "0.**" and then change that into a percentage to display as text in a scene. I'm sure i shouldn't be having this much trouble with it. The issue appears with the line "effiValue = BarScript.FillValue". Thanks in advance

 public class EndScreen : MonoBehaviour
 {
     public BarScript barScript;
      private float effiValue;
      private double effiPercent;
      
 
     void Start()
     {
         
     }
 
     void Update()
     {
          effiValue = BarScript.FillValue;
          effiPercent = effiValue * 100;
         
         this.gameObject.GetComponent<Text>().text = "Efficiency: " + effiValue+ "%";
     }
 }