Question Need help with a tutorial

Hi everyone. Im'm a beginner using Unity and learning codes, so I gave it a try and started making a simple game following this tutorial https://learn.unity.com/tutorial/live-session-making-a-flappy-bird-style-game?signup=true
The thing is, I'm having troubles with the scripts. The guy in the tutorial uses different scripts for videos and website and I'm following the videos.

"NullReferenceException: Object reference not set to an instance of an object
Plane.OnCollisionEnter2D (UnityEngine.Collision2D other) (at Assets/scripts/Plane.cs:35)"

This is the error message. I have fixed previous errors and maybe I made a new one. These are "Plane" "GameController" scripts which have been giving problems :C Thanks to anyone who responds

Code (CSharp):

1. using System.Collections;
2. using System.Collections.Generic;
3. using UnityEngine;
4.  
5. public class Plane : MonoBehaviour
6. {
7.  
8. public float upForce = 300f;
9.  
10.     private bool isDead = false;
11.     private Rigidbody2D rb2d;
12.  
13.     // Start is called before the first frame update
14.     void Start()
15.     {
16.         rb2d = GetComponent<Rigidbody2D> ();
17.     }
18.  
19.     // Update is called once per frame
20.     void Update()
21.     {
22.         if (isDead == false)
23.         {
24.             if (Input.GetMouseButtonDown (0))
25.             {
26.                 rb2d.velocity = Vector2.zero;
27.                 rb2d.AddForce(new Vector2(0, upForce));
28.             }
29.         }
30.     }
31.  
32.     void OnCollisionEnter2D(Collision2D other)
33.     {
34.         isDead = true;
35.         GameControl.instance.PlaneDied ();
36.    
37.     }
38. }
39.  

Code (CSharp):

1. using System.Collections;
2. using System.Collections.Generic;
3. using UnityEngine;
4.  
5. public class GameController : MonoBehaviour
6. {
7.    
8.     public static GameController instance;
9.     public GameObject GameOverText;
10.     public bool gameOver = false;
11.  
12.     // Start is called before the first frame update
13.     void Awake ()
14.     {
15.         if (instance == null)
16.         {
17.             instance = this;
18.         }
19.         else if (instance != this)
20.         {
21.             Destroy (gameObject);
22.         }
23.     }
24.  
25.     // Update is called once per frame
26.     void Update()
27.     {
28.        
29.     }
30.  
31.     public void PlaneDied ()
32.     {
33.         GameOverText.SetActive (true);
34.         gameOver = true;
35.     }
36. }
37.