Saving current progression question

Ok, so I have two string arrays that I want to save and load on game start. All current daily quests are saved into one array (itemsList[]). All completed daily quests are saved in the other array (completedItemsList[]).

I have a Serialzation/Deserialization script for saving and loading the data.
At runtime, the current daily quests (itemsList[]) are loaded just fine. My problem is saving and loading the completed daily quests (completedItemsList[]).
When I click on a game object corresponding to a current daily quest, I want the name of that quest to be added to the completedItemList[] and then save the state. The completed daily quests do not save though.
This is my save/load Serialization/Deserialization code that saved the data to a file in binary.

 using UnityEngine;
 using System.IO;
 using System.Runtime.Serialization.Formatters.Binary;
 
 public static class SaveSystem
 {
     public static void SavePlayer(Player player)
     {
         BinaryFormatter formatter = new BinaryFormatter();
         string path = Application.persistentDataPath + "/1.yummy";
         FileStream stream = new FileStream(path, FileMode.Create);
         playData data = new playData(player);
 
         formatter.Serialize(stream, data);
         stream.Close();
     } 

     public static playData LoadPlayer()
     {
         string path = Application.persistentDataPath + "/1.yummy";
         if (File.Exists(path))
         {
             BinaryFormatter formatter = new BinaryFormatter();
             FileStream stream = new FileStream(path, FileMode.Open);
 
             playData data = formatter.Deserialize(stream) as playData;
             stream.Close();
 
             return data;
         }
         else
         {
             Debug.LogError("Save not found in " + path);
             return null;
         }
     }
 }

This is my data that I want to save and load:

 using System.Collections;
 using System.Collections.Generic;
 using System.Linq;
 using UnityEngine;
 
 [System.Serializable]
 public class playData
 {
     public string[] allItems = new string[Player.itemsList.Count];
     public string[] completedItems = new string[Player.completedItemsList.Count];
     public int containCounter;
     
     public playData (Player player)
     {
         int count = 0;
         foreach(string a in Player.itemsList)
         {
             allItems[count] = a;
             
             count++;
 
         }
         int cc = 0;
         foreach(string c in Player.completedItemsList)
         {
             if(completedItems[count] != null)
             completedItems[count] = c;
             
             cc++;
             
         }
     }
 }

This is where the actual data is saved/loaded:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class Player : MonoBehaviour
 {
     public static List<string> itemsList = new List<string>();
     public static List<string> completedItemsList = new List<string>();
     public static int containCountHolder;
 
 
     public void Start()
     {
         
         playData data = SaveSystem.LoadPlayer();
         
         for (int count = 0; count < 100; count++)
         {
 
             itemsList.Add(data.allItems[count]);
             completedItemsList.Add(data.completedItems[count]);
 
         }
         
     }
     public void Update()
     {
         SaveSystem.SavePlayer(this);
     }
 }

I have an OnClick event for each game object corresponding to each current daily quest string. This onClick event should take the EventSystem.current.currentSelectedGameObject.name, and save that in the completedItemsList[].
When I start game trying to load compeletedItemsList[],, I get an error message like this:

When I load the game WITHOUT the completedItemsList, everything works pretty well.