Bug Null reference error when enemies reached the exit point

Hi guys,
I'm new here, I'm trying to learn unity coding, I have started working on a tower defense game that I found on github.
I'm trying to modify it step by step and learn how the codes works but I get an error everytime I start the game and an enemies reach the final point.

That's the error
NullReferenceException: Object reference not set to an instance of an object
Enemy.OnTriggerEnter2D (UnityEngine.Collider2D collider2D) (at Assets/Scripts/Enemy.cs:73)

I tried to understand why it is null and I think it depends from the way the GameManager was called in the enemy script but I can't find how to fix it.

That's the Enemy script:

Code (CSharp):

1. using System.Collections;
2. using System.Collections.Generic;
3. using UnityEngine;
4.  
5. public class Enemy : MonoBehaviour {
6.     //SerializeField - Allows Inspector to get access to private fields.
7.     //If we want to get access to this from another class, we'll just need to make public getters
8.     [SerializeField]
9.     private Transform exitPoint;
10.     [SerializeField]
11.     private Transform[] wayPoints;
12.     [SerializeField]
13.     private float navigationUpdate;
14.     [SerializeField]
15.     private int healthPoints;
16.     [SerializeField]
17.     private int rewardAmount;
18.  
19.     private int target = 0;
20.     private Transform enemy;
21.     private Collider2D enemyCollider;
22.     private Animator anim;
23.     private float navigationTime = 0;
24.     private bool isDead = false;
25.  
26.     public bool IsDead
27.     {
28.         get { return isDead; }
29.     }
30.  
31.     // Use this for initialization
32.     void Start () {
33.         enemy = GetComponent<Transform>();
34.         enemyCollider = GetComponent<Collider2D>();
35.         anim = GetComponent<Animator>();
36.         Debug.Log("Starting register");
37.         GameManager.Instance.RegisterEnemy(this);
38.         Debug.Log("Registered");
39.     }
40.    
41.     // Update is called once per frame
42.     void Update () {
43.         if (wayPoints != null && !isDead)
44.         {
45.             //Lets use change how fast the update occurs
46.             navigationTime += Time.deltaTime;
47.             if(navigationTime > navigationUpdate)
48.             {
49.                 //If enemy is not at the last wayPoint, keep moving towards the wayPoint
50.                 //otherwise move to the exitPoint
51.                 if(target < wayPoints.Length)
52.                 {
53.                     enemy.position = Vector2.MoveTowards(enemy.position, wayPoints[target].position, navigationTime);
54.                 }
55.                 else
56.                 {
57.                     enemy.position = Vector2.MoveTowards(enemy.position, exitPoint.position, navigationTime);
58.                 }
59.                 navigationTime = 0;
60.             }
61.         }
62.     }
63.  
64.     //If we trigger the collider2D.tag for checkpoints for finish.
65.     //If it hits the checkpoints, increase the index and move to the next checkpoint
66.     //otherwise enemy is at the finish line and should be destroyed.
67.     void OnTriggerEnter2D(Collider2D collider2D)
68.     {
69.         if (collider2D.tag == "checkpoint")
70.             target += 1;
71.         else if (collider2D.tag == "Finish")
72.         {
73.             GameManager.Instance.RoundEscaped += 1;
74.             GameManager.Instance.TotalEscape += 1;
75.             GameManager.Instance.UnregisterEnemy(this);
76.             GameManager.Instance.isWaveOver();
77.         }
78.         else if(collider2D.tag == "projectile")
79.         {
80.             Projectile newP = collider2D.gameObject.GetComponent<Projectile>();
81.             enemyHit(newP.AttackStrength);
82.             Destroy(collider2D.gameObject);
83.         }
84.     }
85.     public void enemyHit(int hitPoints)
86.     {
87.         if(healthPoints - hitPoints > 0)
88.         {
89.             healthPoints -= hitPoints;
90.             anim.Play("Hurt");
91.             GameManager.Instance.AudioSource.PlayOneShot(SoundManager.Instance.Hit);
92.         }
93.         else
94.         {
95.             anim.SetTrigger("didDie");
96.             die();
97.         }
98.     }
99.  
100.     public void die()
101.     {
102.         isDead = true;
103.         enemyCollider.enabled = false;
104.         GameManager.Instance.TotalKilled += 1;
105.         GameManager.Instance.AudioSource.PlayOneShot(SoundManager.Instance.Death);
106.         GameManager.Instance.AddMoney(rewardAmount);
107.         GameManager.Instance.isWaveOver();
108.     }
109. }

All the things in this condition else if (collider2D.tag == "Finish") (line 71) seems to doesn't work properly.

That's the GameManager script

Code (CSharp):

1. using System.Collections;
2. using System.Collections.Generic;
3. using UnityEngine;
4. using UnityEngine.UI;
5.  
6. public enum gameStatus
7. {
8.     next, play, gameover, win
9. }
10. public class GameManager : Singleton<GameManager> {
11.     //SerializeField - Allows Inspector to get access to private fields.
12.     //If we want to get access to this from another class, we'll just need to make public getters
13.     [SerializeField]
14.     private int totalWaves = 10;
15.     [SerializeField]
16.     private Text totalMoneyLabel;   //Refers to money label at upper left corner
17.     [SerializeField]
18.     private Text currentWaveLabel;
19.     [SerializeField]
20.     private Text totalEscapedLabel;
21.     [SerializeField]
22.     private GameObject spawnPoint;
23.     [SerializeField]
24.     private Enemy[] enemies;
25.     [SerializeField]
26.     private int totalEnemies = 3;
27.     [SerializeField]
28.     private int enemiesPerSpawn;
29.     [SerializeField]
30.     private Text playButtonLabel;
31.     [SerializeField]
32.     private Button playButton;
33.  
34.     private int waveNumber = 0;
35.     private int totalMoney = 10;
36.     private int totalEscaped = 0;
37.     private int roundEscaped = 0;
38.     private int totalKilled = 0;
39.     private int whichEnemiesToSpawn = 0;
40.     private int enemiesToSpawn = 0;
41.     private gameStatus currentState = gameStatus.play;
42.     private AudioSource audioSource;
43.  
44.     public List<Enemy> EnemyList = new List<Enemy>();
45.     const float spawnDelay = 2f; //Spawn Delay in seconds
46.  
47.     public int TotalMoney
48.     {
49.         get { return totalMoney; }
50.         set
51.         {
52.             totalMoney = value;
53.             totalMoneyLabel.text = totalMoney.ToString();
54.         }
55.     }
56.  
57.     public int TotalEscape
58.     {
59.         get { return totalEscaped; }
60.         set { totalEscaped = value; }
61.     }
62.     public int RoundEscaped
63.     {
64.         get { return roundEscaped; }
65.         set { roundEscaped = value; }
66.     }
67.     public int TotalKilled
68.     {
69.         get { return totalKilled; }
70.         set { totalKilled = value; }
71.     }
72.  
73.     public AudioSource AudioSource
74.     {
75.         get { return audioSource; }
76.     }
77.    
78.     // Use this for initialization
79.     void Start () {
80.         playButton.gameObject.SetActive(false);
81.         audioSource = GetComponent<AudioSource>();
82.         ShowMenu();
83.     }
84.    
85.     // Update is called once per frame
86.     void Update () {
87.         handleEscape();
88.     }
89.  
90.     //This will spawn enemies, wait for the given spawnDelay then call itself again to spawn another enemy
91.     IEnumerator spawn()
92.     {
93.         if (enemiesPerSpawn > 0 && EnemyList.Count < totalEnemies)
94.         {
95.             for (int i = 0; i < enemiesPerSpawn; i++)
96.             {
97.                 if (EnemyList.Count < totalEnemies)
98.                 {
99.                     Enemy newEnemy = Instantiate(enemies[Random.Range(0, enemiesToSpawn)]);
100.                     newEnemy.transform.position = spawnPoint.transform.position;
101.                 }
102.             }
103.             yield return new WaitForSeconds(spawnDelay);
104.             StartCoroutine(spawn());
105.         }
106.     }
107.  
108.     ///Register - when enemy spawns
109.     public void RegisterEnemy(Enemy enemy)
110.     {
111.         EnemyList.Add(enemy);
112.     }
113.     ///Unregister - When they escape the screen
114.     public void UnregisterEnemy(Enemy enemy)
115.     {
116.         EnemyList.Remove(enemy);
117.         Destroy(enemy.gameObject);
118.     }
119.     ///Destroy - At the end of the wave
120.     public void DestroyAllEnemies()
121.     {
122.         foreach(Enemy enemy in EnemyList)
123.         {
124.             Destroy(enemy.gameObject);
125.         }
126.         EnemyList.Clear();
127.     }
128.  
129.     public void AddMoney(int amount)
130.     {
131.         TotalMoney += amount;
132.     }
133.  
134.     public void SubtractMoney(int amount)
135.     {
136.         TotalMoney -= amount;
137.     }
138.  
139.     public void isWaveOver()
140.     {
141.         totalEscapedLabel.text = "Escaped " + TotalEscape + "/10";
142.         if (RoundEscaped + TotalKilled == totalEnemies)
143.         {
144.             if(waveNumber <= enemies.Length)
145.             {
146.                 enemiesToSpawn = waveNumber;
147.             }
148.             setCurrentGameState();
149.             ShowMenu();
150.         }
151.     }
152.  
153.     public void setCurrentGameState()
154.     {
155.         if(totalEscaped >= 10)
156.         {
157.             currentState = gameStatus.gameover;
158.         }
159.         else if(waveNumber == 0 && (TotalKilled + RoundEscaped) == 0)
160.         {
161.             currentState = gameStatus.play;
162.         }
163.         else if(waveNumber >= totalWaves)
164.         {
165.             currentState = gameStatus.win;
166.         }
167.         else
168.         {
169.             currentState = gameStatus.next;
170.         }
171.     }
172.  
173.     public void ShowMenu()
174.     {
175.         switch (currentState)
176.         {
177.             case gameStatus.gameover:
178.                 playButtonLabel.text = "Play Again!";
179.                 AudioSource.PlayOneShot(SoundManager.Instance.Gameover);
180.                 break;
181.             case gameStatus.next:
182.                 playButtonLabel.text = "Next Wave";
183.                 break;
184.             case gameStatus.play:
185.                 playButtonLabel.text = "Play";
186.                 break;
187.             case gameStatus.win:
188.                 playButtonLabel.text = "Play";
189.                 break;
190.         }
191.         playButton.gameObject.SetActive(true);
192.     }
193.     public void playButtonPressed()
194.     {
195.         Debug.Log("Play Button Pressed");
196.         switch (currentState)
197.         {
198.             case gameStatus.next:
199.                 waveNumber += 1;
200.                 totalEnemies += waveNumber;
201.                 break;
202.             default:
203.                 totalEnemies = 3;
204.                 totalEscaped = 0;
205.                 TotalMoney = 10;
206.                 TowerManager.Instance.DestroyAllTower();
207.                 TowerManager.Instance.RenameTagsBuildSites();
208.                 totalMoneyLabel.text = TotalMoney.ToString();
209.                 totalEscapedLabel.text = "Escaped " + totalEscaped + "/10";
210.                 AudioSource.PlayOneShot(SoundManager.Instance.NewGame);
211.                 break;
212.         }
213.         DestroyAllEnemies();
214.         TotalKilled = 0;
215.         RoundEscaped = 0;
216.         currentWaveLabel.text = "Wave " + (waveNumber + 1);
217.         StartCoroutine(spawn());
218.         playButton.gameObject.SetActive(false);
219.     }
220.     private void handleEscape()
221.     {
222.         if (Input.GetKeyDown(KeyCode.Escape))
223.         {
224.             TowerManager.Instance.disableDragSprite();
225.             TowerManager.Instance.towerButtonPressed = null;
226.         }
227.     }
228.  
229. }

Thank you guys for the help

 

What is this "Singleton" class? I do not recognize it.

 

Line 73 is this:

GameManager.Instance.RoundEscaped += 1;

So that tells us that GameManager.Instance is null. There are two possibilities- either GameManager.Instance is never assigned a value before this line executes, or else the object that GameManager.Instance refers to is deleted at some point.

 

Thank you, I will have a look