I wonder if we can create a List can remain even when we change scene or reopen game?

I'm learning Unity by creating a simple game, it's pretty done, but I want to create a Rank List that can contain top5 bestscore that Player ever had at an object that can't be destroyed when i'm change game scene, each time I push the "add" button, It'll check and save as a ranktop, I tried like this:

 public class BestScorestore : MonoBehaviour
 {
     public static BestScorestore Main;
     public List<Ranking> rankstorage;
     private const string bestScore = "Best Score";
     private const string Playername = "Player's name";
     public Ranking ranktop;
     private void Awake()
     {
         Singleconnect();
         isFirstrun();
     }
     void isFirstrun()
     {
         if (!PlayerPrefs.HasKey("isFirstrun"))
         {
             PlayerPrefs.SetInt(bestScore, 0);
             PlayerPrefs.SetInt("isFirstrun", 0);
             rankstorage = new List<Ranking>();
         }
     }
 void Singleconnect()
     {
         if (Main != null)
         {
             Destroy(gameObject);
         }
         else
         {
             Main = this;
             DontDestroyOnLoad(gameObject);
         }
     }
 public void checkRank(string nametop,int scoretop)
     {
         ranktop = new Ranking(nametop, scoretop);
         if (rankstorage.Count > 0)
         {
             for (int i = 0; i < rankstorage.Count; i++)
             {
                 if (ranktop.rankscore > rankstorage[i].rankscore)
                 {
                     rankstorage.Insert(i, ranktop);
                 }
             }
             if (rankstorage.Count > 5)
             {
                 for (int k = rankstorage.Count - 1; k > 4; k--)
                 {
                     rankstorage.Remove(rankstorage[k]);
                 }
             }
         }
         else{
             rankstorage.Add(ranktop);
         }
     }
 public void setPlayerName(string name)
     {
         PlayerPrefs.SetString(Playername, name);
     }
     public string getPlayerName()
     {
         return PlayerPrefs.GetString(Playername);
     }
 public void setBestScore(int score)
     {
         PlayerPrefs.SetInt(bestScore, score);
     }
     public int getBestScore()
     {
         return PlayerPrefs.GetInt(bestScore);
     }
 }
 public class Ranking
 {
     public string rankname;
     public int rankscore;
     public Ranking(string name, int score)
     {
         rankname = name;
         rankscore = score;
     }
 }

and in the GameManage's script i used like this one

 {
     [SerializeField]
     private List<Text> topname=new List<Text>();
     [SerializeField]
     private List<Text> topscore=new List<Text>();
     [SerializeField]
     private Text currentName, currentScore;
     private Ranking currentplayer;
     private string currentname;
     private int currentscore;
     void ShowRankList()
     {
         foreach (Text child in topname)
     {
         child.text = "";
     }
     foreach (Text child in topscore)
     {
         child.text = "";
     }
     currentname = BestScorestore.Main.getPlayerName();
     currentscore = BestScorestore.Main.getBestScore();
     currentName.text = "" + currentname;
     currentScore.text = "" + currentscore;
     var m = BestScorestore.Main;
     if (m.rankstorage == null) return;
     for (int i = 0; i < m.rankstorage.Count - 1; i++)
     {
         topname[i].text = m.rankstorage[i].rankname;
         topscore[i].text = "" + m.rankstorage[i].rankscore;
     }
     }
     public void addButton()
     {
         k.Clicksound();
         currentname = currentName.text;
     currentplayer = new Ranking(currentname, currentscore);
     BestScorestore.Main.setPlayerName(currentplayer.rankname);
     BestScorestore.Main.checkRank(currentplayer.rankname,currentplayer.rankscore);
     if (!BestScorestore.Main.rankstorage.Contains(currentplayer))
     {
         Debug.Log("Your score is too low to put in ranklist");
     }
     else Debug.Log("Add!");
     addScorePannel.SetActive(false);
     buttonPlus.GetComponent<Button>().enabled = true;
     foreach (GameObject child in k.closeButtons)
     {
         child.GetComponent<Button>().enabled = true;
     }
     ShowRankList();
     }
 }

but when i open Rank list, and push the "Add" button, it show:

 "NullReferenceException: Object reference not set to an instance of an object"

when i check rankstorage.count at my checkRank(string nametop,int scoretop)

Am I doing wrong somewhere? Please help!

I'm sorry that i wrote too much in there...