Object reference not set to an instance of an object

I don't really know what's wrong. Here's my code

 //HighscoreScript
 using UnityEngine;
 using System.Collections;
 
 public class HighscoreScript : MonoBehaviour {
     
     const string privateCode = "private code";
     const string publicCode = "548c04c96e51b60c740bcdec";
     const string webURL = "http://dreamlo.com/lb/";
     
     DisplayHighscores highscoreDisplay;
     public static Highscore[] highscoresList;
     static HighscoreScript instance;
     
     void Awake() {
         instance = this;
         highscoreDisplay = GetComponent<DisplayHighscores> ();
     }
     
     public static void AddNewHighscore() {
         Debug.Log ("Called");
         instance.StartCoroutine(instance.UploadNewHighscore());
     }
 
     IEnumerator UploadNewHighscore() {
         WWW www = new WWW(webURL + privateCode + "/add/" + WWW.EscapeURL(SystemInfo.deviceName) + "/" + SpeedController.score);
         yield return www;
         
         if (string.IsNullOrEmpty(www.error)) {
             print ("Upload Successful");
             DownloadHighscores();
         }
         else {
             print ("Error uploading: " + www.error);
         }
     }
     
     public void DownloadHighscores() {
         StartCoroutine("DownloadHighscoresFromDatabase");
     }
     
     IEnumerator DownloadHighscoresFromDatabase() {
         WWW www = new WWW(webURL + publicCode + "/pipe/");
         yield return www;
         
         if (string.IsNullOrEmpty (www.error)) {
             FormatHighscores (www.text);
             highscoreDisplay.OnHighscoresDownloaded(highscoresList);
         }
         else {
             print ("Error Downloading: " + www.error);
         }
     }
     
     void FormatHighscores(string textStream) {
         string[] entries = textStream.Split(new char[] {'\n'}, System.StringSplitOptions.RemoveEmptyEntries);
         highscoresList = new Highscore[entries.Length];
         
         for (int i = 0; i <entries.Length; i ++) {
             string[] entryInfo = entries[i].Split(new char[] {'|'});
             string username = entryInfo[0];
             int score = int.Parse(entryInfo[1]);
             highscoresList[i] = new Highscore(username,score);
         }
     }
     
 }
 
 public struct Highscore {
     public string username;
     public int score;
     
     public Highscore(string _username, int _score) {
         username = _username;
         score = _score;
     }    
 }

The error says NullReferenceException: Object reference not set to an instance of an object HighscoreScript.AddNewHighscore () (at Assets/Scripts/HighscoreScript.cs:25)