Unity: Loading class instances from Json file gives ArgumentNullException

Question

In my game, I have a Computer class with following variables:

 [System.Serializable]
 public class Computer : MonoBehaviour, ShopItem{
     public string model;
     public int price;
     public int speed;
     public int watt;
     public Sprite image;
 }

And I have a script that contains a list of Computer:

   List<Computer> computerList;

Then when I save the game, I pass all data to the DataClass and convert it to Json:

 public string SaveToJson(){
     //Create data class
     DataClass dataClass = new DataClass(...other parameters..., computerList);
     //Convert to json
     string jsonString = JsonUtility.ToJson(dataClass, false);
     //Save to PlayerPref
     PlayerPrefs.SetString("GameData", jsonString);
 }

I also stored it into a json file just to see the format. The computerList contains some instanceID like below.

 "computerList": [
         {
             "instanceID": 655388
         },
         {
             "instanceID": 654696
         },
         {
             "instanceID": 655502
         },
         {
             "instanceID": 654696
         }
     ],

When load game,I load the playerPref and convert it back to dataClass:

 public void LoadGame(){
     string result = PlayerPrefs.GetString("GameData");
     DataClass dataClass = JsonUtility.FromJson<DataClass>(result);  
     this.computerList = dataClass.computerList;
 }

But when I try to access the computerList, it gives me this error: ArgumentNullException: Argument cannot be null.

I also saved it in the localhost database, and when I load it back it gives the same error, so I guess it's nothing wrong about PlayerPrefs. Other variables like int, float, list does well, so the problem should be about custom classes. Any help would be appreciate.

The full error code is:

ArgumentNullException: Argument cannot be null. Parameter name: key System.Collections.Generic.Dictionary`2[ShopItem,System.Int32].ContainsKey (ShopItem key) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Collections.Generic/Dictionary.cs:280) InventoryManager.ProcessItemList () (at Assets/Script/CoreGame/Local Manager/InventoryManager.cs:54) InventoryManager.ShowInvnetory () (at Assets/Script/CoreGame/Local Manager/InventoryManager.cs:89) UnityEngine.Events.InvokableCall.Invoke () (at C:/buildslave/unity/build/Runtime/Export/UnityEvent.cs:166) UnityEngine.Events.UnityEvent.Invoke () (at C:/buildslave/unity/build/Runtime/Export/UnityEvent_0.cs:58) UnityEngine.UI.Button.Press () (at C:/buildslave/unity/build/Extensions/guisystem/UnityEngine.UI/UI/Core/Button.cs:66) UnityEngine.UI.Button.OnPointerClick (UnityEngine.EventSystems.PointerEventData eventData) (at C:/buildslave/unity/build/Extensions/guisystem/UnityEngine.UI/UI/Core/Button.cs:108) UnityEngine.EventSystems.ExecuteEvents.Execute (IPointerClickHandler handler, UnityEngine.EventSystems.BaseEventData eventData) (at C:/buildslave/unity/build/Extensions/guisystem/UnityEngine.UI/EventSystem/ExecuteEvents.cs:50) UnityEngine.EventSystems.ExecuteEvents.Execute[IPointerClickHandler] (UnityEngine.GameObject target, UnityEngine.EventSystems.BaseEventData eventData, UnityEngine.EventSystems.EventFunction`1 functor) (at C:/buildslave/unity/build/Extensions/guisystem/UnityEngine.UI/EventSystem/ExecuteEvents.cs:261) UnityEngine.EventSystems.EventSystem:Update()