Nullreferenceexception: object reference not set to an instance of an object | but nothing is null! ?

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C#

null

Unity

Unity3D

Please read before this question is reported / taken offline! This question has already been taken offline on StackOverflow and I was pointed to the resolution of a general NullReferenceException. But these solutions somehow don't work for me 😥. Also, I'm a beginner 😅.

I get the well-known 'NullReferenceException: Object reference not set to an instance of an object' error. The detailed description: 'Coin.OnTriggerEnter2D (UnityEngine.Collider2D col) (at Assets/Space Adventure/Scripts/Game/Coin.cs:34)'.

These are my two scripts:

Coin Script (where I try to execute the SendBitcoin):

using UnityEngine;
 
public class Coin : MonoBehaviour
{
    private Animation anim;
    private AudioSource audioSource;
    private bool taken;
 
    void Start()
    {
        anim = this.GetComponent<Animation>();
        audioSource = this.GetComponent<AudioSource>();
        taken = false;
    }
 
    void OnTriggerEnter2D(Collider2D col)
    {
        // Checks if coin has been taken. (Because player can re-enter coin while coin destroy animation is playing)
        if(!taken)
        {
            // Checks if coin collides with one of the player colliders.
            if(col.name == "Front" || col.name == "Back")
            {
                // Increases player wallet amount.
                Wallet.SetAmount(Wallet.GetAmount() + 1);
                // Play coin destroy animation.
                anim.Play("Coin-Destroy-Down");
                // Play coin collect sound.
                audioSource.Play();
                // Sets coin as taken.
                taken = true;
               
                // SEND BITCOIN **(here is line 34 and apparently the error)**
                BitcoinController.Instance.SendBitcoin();
            }
        }
    }
}

Bitcoin Controller (where the Bitcoin should be send):

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.Networking;
 
public class GamerTagSend {
    public string gamertag;
    public string amount;
    public string description;
}
 
public class BitcoinController : MonoBehaviour
{
    public static BitcoinController Instance;
 
    private void Awake()
    {
        Instance = this;
        DontDestroyOnLoad(this.gameObject);
    }
 
    public void SendBitcoin()
    {
        StartCoroutine(SendBitcoinCont());
    }
 
    IEnumerator SendBitcoinCont()
    {
        UnityWebRequest request = new UnityWebRequest("[here is the url]", "POST");
 
        GamerTagSend postData = new GamerTagSend();
        postData.gamertag = "gamertag";
        postData.amount = "1";
        postData.description = "It works : )";
        string myjson = JsonUtility.ToJson(postData);
        byte[] postDataBytes = System.Text.Encoding.UTF8.GetBytes(myjson);
 
        request.downloadHandler = (DownloadHandler)new DownloadHandlerBuffer();
        request.uploadHandler = (UploadHandlerRaw)new UploadHandlerRaw(postDataBytes);
 
        request.SetRequestHeader("Content-Type", "application/json");
        request.SetRequestHeader("apikey", "[here is my api key]");
 
        yield return request.SendWebRequest();
 
        if (request.isNetworkError || request.isHttpError)
        {
            Debug.LogError("Error " + request.downloadHandler.text);
        }
        else
        {
            Debug.Log("sent!");
        }
    }
}

I hope you can help me. Thanks in advance. I've been trying to solve this problem for ages. I don't get it.

Please suggest code changes.

What I have tried:

Asking on StackOverflow (removed), searching the web, and searching the official Unity documentation.