What is a NullRefrenceException and how do I fix it?

So I'm trying to create a health bar for my game, and I have 2 scripts I'm referencing to get it to work. The one managing the UI is tied to the bar itself

Code (CSharp):

1. using System.Collections;
2. using System.Collections.Generic;
3. using UnityEngine;
4. using UnityEngine.UI;
5.  
6. public class HealthBar : MonoBehaviour
7. {
8.    public Slider slider;
9.  
10.    public void SetMaxHealth(int health)
11.    {
12.        slider.maxValue = health;
13.        slider.value = health;
14.    }
15.  
16.    public void SetHealth(int health)
17.    {
18.        slider.value = health;
19.    }
20. }
21.  

and the one managing the health is tied to the player

Code (CSharp):

1. using System.Collections;
2. using System.Collections.Generic;
3. using UnityEngine;
4.  
5. public class PlayerStats : MonoBehaviour
6. {
7.       public int maxHealth = 100;
8.       public int currentHealth;
9.       public HealthBar healthBar;
10.     void Start()
11.     {
12.        currentHealth = maxHealth;
13.        healthBar.SetMaxHealth(maxHealth);
14.     }
15.  
16.     void Update()
17.     {
18.        if (Input.GetKeyDown(KeyCode.Space))
19.        {
20.          TakeDamage(20);
21.        }
22.     }
23.     void TakeDamage(int damage)
24.     {
25.         currentHealth -= damage;
26.         healthBar.SetHealth(currentHealth);
27.     }
28. }

Basically, I'm testing to see the health bar in action by removing hitpoints with the space key. Problem is that nothing is happening to the UI, my player is taking damage, and the debug console is flooded with said errors. Everything but the slider is working fine. Help would be much appreciated, especially a solution.

 

nullreference means you're trying to use a property that doesn't reference an object. I assume you haven't assigned healthBar in the inspector and all the null references are from where you use them.

 

Could you explain? I'm not sure how to do that. As far as I can tell my script is active and assigned to the health bar, or are you saying I should drag it to the character?

 

My brain does not like to function sometimes. Thank you for your help, I've solved it lol.