2D Platformer - Problem with object that kills, then restarts after build

Hiya. I'm new to Unity and C# and have been following a youtuber's tutorials on making a 2D endless runner game. In Unity, everything works perfectly, but when I build it. the 'deathzone' doesn't work. I built it with developer version and this is what comes up:

 UnityException: GameObject has undefined tag!

I don't know how I'm meant to fix it. The code is below, or here: http://collabedit.com/42yp8

For the player controller:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class Controls : MonoBehaviour {
 
     public float moveSpeed;
     private float moveSpeedStore;
     public float speedMultiplier;
 
     public float speedIncreaseMilestone;
     private float speedIncreaseMilestoneStore;
 
     private float speedMilestoneCount;
     private float speedMilestoneCountStore;
 
     public float jumpForce;
 
     public float jumpTime;
     private float jumpTimeCounter;
 
     private Rigidbody2D myRigidbody;
 
     public bool grounded;
     public LayerMask whatIsGround;
 
     private Collider2D myCollider;
 
     private Animator myAnimator;
 
     public GameManager theGameManager;
 
     // Use this for initialization
     void Start () {
         myRigidbody = GetComponent<Rigidbody2D>();
 
         myCollider = GetComponent<Collider2D>();
 
         myAnimator = GetComponent<Animator>();
 
         jumpTimeCounter = jumpTime;
 
         moveSpeedStore = moveSpeed;
         speedMilestoneCountStore = speedMilestoneCount;
         speedIncreaseMilestoneStore = speedIncreaseMilestone;
     }
 
     // Update is called once per frame
     void Update() {
 
         grounded = Physics2D.IsTouchingLayers(myCollider, whatIsGround);
 
         if(transform.position.x > speedMilestoneCount)
         {
             speedMilestoneCount += speedIncreaseMilestone;
 
             speedIncreaseMilestone = speedIncreaseMilestone * speedMultiplier;
 
             moveSpeed = moveSpeed * speedMultiplier;
         }
 
         myRigidbody.velocity = new Vector2(moveSpeed, myRigidbody.velocity.y);
 
         if (Input.GetKeyDown(KeyCode.W) || Input.GetKeyDown(KeyCode.Space) || Input.GetKeyDown(KeyCode.UpArrow) || Input.GetMouseButtonDown(0))
         {
             if (grounded)
             {
                 myRigidbody.velocity = new Vector2(myRigidbody.velocity.x, jumpForce);
             }
         }
 
         if (Input.GetKey(KeyCode.Space) || Input.GetKey(KeyCode.W) || Input.GetKeyDown(KeyCode.UpArrow) || Input.GetMouseButton(0))
         {
             if(jumpTimeCounter > 0)
             {
                 myRigidbody.velocity = new Vector2(myRigidbody.velocity.x, jumpForce);
                 jumpTimeCounter -= Time.deltaTime;
             }
         }
 
         if (Input.GetKeyUp(KeyCode.Space) || Input.GetMouseButtonUp(0) || Input.GetKeyDown(KeyCode.UpArrow) || Input.GetKeyUp(KeyCode.W))
         {
             jumpTimeCounter = 0;
         }
 
         if(grounded)
         {
             jumpTimeCounter = jumpTime;
         }
 
         myAnimator.SetFloat("Speed", myRigidbody.velocity.x);
         myAnimator.SetBool("Grounded", grounded);
     }
 
     void OnCollisionEnter2D(Collision2D other)
     {
         if (other.gameObject.tag == "killbox")
         {
             theGameManager.RestartGame();
             moveSpeed = moveSpeedStore;
             speedMilestoneCount = speedMilestoneCountStore;
             speedIncreaseMilestone = speedIncreaseMilestoneStore;
         }
     }
 }
 
 
 
 
 
 
 
 
 
 
 
 

Game Manager

 using System.Collections.Generic;
 using System.Collections;
 using UnityEngine;
 using UnityEngine.SceneManagement;
 
 public class GameManager : MonoBehaviour {
 
     public Transform platformGenerator;
     private Vector3 platformStartPoint;
 
     public Controls thePlayer;
     private Vector3 playerStartPoint;
 
     private PlatformKiller[] platformList;
 
     private ScoreManager theScoreManager;
 
     // Use this for initialization
     void Start ()
     {
         platformStartPoint = platformGenerator.position;
         playerStartPoint = thePlayer.transform.position;
 
         theScoreManager = FindObjectOfType<ScoreManager>();
     }
     
     // Update is called once per frame
     void Update ()
     {
         
     }
 
     public void RestartGame()
     {
         StartCoroutine("RestartGameCo");
     }
 
     public IEnumerator RestartGameCo()
     {
         theScoreManager.scoreIncreasing = false;
         thePlayer.gameObject.SetActive(false);
         yield return new WaitForSeconds(0.5f);
         platformList = FindObjectsOfType<PlatformKiller>();
         for(int i = 0; i < platformList.Length; i++)
         {
             platformList[i].gameObject.SetActive(false);
         }
 
         thePlayer.transform.position = playerStartPoint;
         platformGenerator.position = platformStartPoint;
         thePlayer.gameObject.SetActive(true);
 
         theScoreManager.scoreCount = 0;
         theScoreManager.scoreIncreasing = true;
     }
 }