Making a game where objects need to be shot off of a surface. level complete when all objects are shot off. difficulty in scripting for level completion.

this is the script I am using: using System.Collections; using System.Collections.Generic; using UnityEngine;

public class LevelComplete : MonoBehaviour {

 private GameObject target;
 
 private void Start()
 {
     target = GameObject.FindWithTag("Obstacle");
     
 }
 
 // Update is called once per frame
 void Update()
 {
     List<float> obstaclePosition = new List<float>();
     obstaclePosition.Add(target.transform.position.y);
     for( int i = 0; i<obstaclePosition.Count;i++);
     {
         if (obstaclePosition[i]<-3)
         {
         Debug.Log("level complete");
         }

     }

my understanding:

1. obstaclePosition is a list that contains all y axis position of objects tagged as obstacle

2. as it is updated every frame , it will always have number of elements equal to objects tagged obstacle

3. when all elements are lesser than -3 , console should display level complete

problems: 1. Null Reference Exception 2. Will my program work as intended after the null reference exception is sorted ?