NullReferenceException

NullReferenceException: Object reference not set to an instance of an object
PlayerHealthManager.HurtPlayer (Int32 damageToGive) (at Assets/Scripts/Player/PlayerHealthManager.cs:91)
HurtPlayer.OnCollisionEnter2D (UnityEngine.Collision2D other) (at Assets/Scripts/Enemy/HurtPlayer.cs:22)

Code (CSharp):

1. using System.Collections;
2. using System.Collections.Generic;
3. using UnityEngine;
4. using UnityEngine.SceneManagement;
5. using UnityEngine.UI;
6.  
7. public class PlayerHealthManager : MonoBehaviour {
8.  
9.     // Options that we will be able to change in Unity with sliders:
10.     public int startingHealth;                                      
11.     public int playerCurrentHealth;                                        
12.     public Slider healthSlider;                                                
13.     public Image damageImage;                                                
14.     public AudioClip deathClip;                                                
15.     public float flashspeed = 5f;                                          
16.     public Color flashColour = new Color(1f, 0f, 0f, 0.1f);                
17.  
18.     Animator anim;                                                          
19.     AudioSource playerAudio;                                              
20.     PlayerMovement PlayerMovement;                                            
21.     bool isDead;                                                          
22.     bool damaged;                                                          
23.     [HideInInspector] public bool knockedBack = false;                        
24.     bool knockedUp = false;
25.     bool knockTowardsRight;
26.     [Header ("Knockback")]
27.     public float knockbackForce = 8.0f;
28.     public float knockupForce = 5.0f;
29.     public float knockbackTime = 0.4f;
30.  
31.     void Awake ()
32.     {
33.         anim = GetComponent <Animator> ();
34.         playerAudio = GetComponent <AudioSource>();
35.         PlayerMovement = GetComponent <PlayerMovement> ();
36.        
37.     }
38.  
39.     // Use this for initialization
40.     void Start () {
41.         playerCurrentHealth = startingHealth;  
42.     }
43.    
44.     // Update is called once per frame
45.     void Update () {
46.         if(playerCurrentHealth < 0)
47.         {
48.             // gameObject.SetActive(false);
49.  
50.             // Loads current scene if player dies
51.               SceneManager.LoadScene (SceneManager.GetActiveScene().buildIndex);
52.            
53.  
54.  
55.         }
56.  
57.         // Tells the game which way the characters should be thrown when hit by an enemy.
58.         if (knockedBack)
59.         {
60.             if (knockTowardsRight)
61.             {
62.                 transform.position -= new Vector3 (knockbackForce * Time.deltaTime, 0, 0);
63.             }
64.             else
65.             {
66.                 transform.position += new Vector3 (knockbackForce * Time.deltaTime, 0, 0);
67.             }
68.            
69.             if (knockedUp)
70.             {
71.                 transform.position += new Vector3 (0, knockupForce * Time.deltaTime, 0);
72.             }
73.             else if (!GetComponent <PlayerMovement> ().controller.m_Grounded)
74.             {
75.                 transform.position -= new Vector3 (0, knockupForce * Time.deltaTime, 0);
76.             }
77.         }
78.     }
79.  
80.  
81.     // Makes it possible to decide how much health the player should have
82.     public void HurtPlayer(int damageToGive)
83.         {
84.         // Set the damaged flag so the screen will flash.
85.         damaged = true;
86.  
87.         // Reduce the current health by the damage amount.
88.         playerCurrentHealth -= damageToGive;
89.  
90.         // Set the health bar's value to the current health.
91.         healthSlider.value = playerCurrentHealth;
92.  
93.         // Play the hurt sound effect.
94.         playerAudio.Play ();
95.  
96.         // If the player has lost all it's health and the death flag hasn't been set yet...
97.         if (playerCurrentHealth <= 0 && !isDead);
98.        
99.         // ... it should die.
100.         Death ();
101.            
102.         }
103.  
104.         void Death ()
105.         {
106.             // Set the death flag so this function won't be called again.
107.             isDead = true;
108.  
109.             // Tell the animator that the player is dead.
110.             anim.SetTrigger ("Die");
111.  
112.             // Set the audiosource to play the death clip and play it (this will stop the hurt sound from playing).
113.             playerAudio.clip = deathClip;
114.             playerAudio.Play ();
115.  
116.             // Turn off the movement and shooting scripts.
117.                PlayerMovement.enabled = false;
118.         }
119.  
120.     //
121.     void OnCollisionEnter2D (Collision2D col)
122.     {
123.         if (col.gameObject.tag    == "AI")
124.             {
125.                 knockedBack = true;
126.                 knockedUp = true;
127.  
128.                 if (col.transform.position.x > transform.position.x)
129.                 {
130.                     knockTowardsRight = true;
131.                 }
132.                 else
133.                 {
134.                     knockTowardsRight = false;
135.                 }
136.  
137.                 Invoke ("StopKnockback", knockbackTime);
138.                 Invoke ("StopKnockup", knockbackTime / 2);
139.             }
140.     }
141.  
142.     void StopKnockback ()
143.     {
144.         knockedBack = false;
145.     }
146.  
147.         void StopKnockup ()
148.     {
149.         knockedUp = false;
150.     }
151. }
152.  
153.     // // If the player has just been damaged...
154.     // if(damaged)
155.     // {
156.     //     // ... set the colour of the damageImage to the flash colour.
157.     //     damageImage.color = flashColour;
158.     // }
159.     // // Otherwise...
160.     // else
161.     // {
162.     //      // ... transition the colour back to clear.
163.     //     damageImage.color = Color.Lerp (damageImage.color, Color.clear, flashspeed * Time.deltaTime);
164.     // }
165.     // // Reset the damaged flag.
166.     // damaged = false;
167.  

 

I am trying to find the solution, but nothing is showing up when I open the script. Have also tried using different links within the scripts and naming stuff differently but this one persists.

 

You probably forgot to link healthSlider in your scene.