Can't find instance for Serializable class public property.

i think this has something to do with Serializable but i don't know enough to say how to fix this.

here is my script: using System.Collections; using System.Collections.Generic; using UnityEngine; using UnityEngine.UI;

public class ControlPanel : MonoBehaviour {

 /* 
  * 
  * 
  *  c  l  a  s  s  e  s
  * 
  * 
  */


  public abstract class BoolControl {
     protected string playerprefName;
     public Text buttonText;
     public Button btn;
     public bool defaultState;
     protected bool state {
         get
         {
             return state;
         }
         set {
             if(value){
                 buttonText.text = "OFF";
                 ColorBlock cb = btn.colors;
                 cb.normalColor = Color.blue;
                 btn.colors = cb;
             }else{
                 buttonText.text = "ON";
                 ColorBlock cb = btn.colors;
                 cb.normalColor = Color.black;
                 btn.colors = cb;
             }
             state = value;
         }
     }

     public BoolControl () {
         btn.onClick.AddListener(()=>ToggleState());
         Debug.LogError("added event listener");
     }

     public void ToggleState()
     {
         Debug.LogError("State change");
        
         state = !state;
         ApplyChanges();
     }

     public abstract void ApplyChanges();
     
     public void SavePlayerpref()
     {
         PlayerPrefs.SetInt(playerprefName, System.Convert.ToInt32(state));
     }
     public void ReadPlayerpref()
     {
         state = System.Convert.ToBoolean(PlayerPrefs.GetInt(playerprefName, System.Convert.ToInt32(defaultState)));
     }
 }

 [System.Serializable]
 public class Syphon : BoolControl
 {
     public syphonController script;

     public Syphon():base(){
         playerprefName = "SyphonState";
     }

     public override void ApplyChanges(){
         
     }
 }

 [System.Serializable]
 public class Hue : BoolControl {
     
     public HueLightController script;


     public Hue():base(){
         playerprefName = "HueState";
     }

     public override void ApplyChanges()
     {
         script.on = state;
         if(state){
             
         }else{
             
         }
     }
 }

 [System.Serializable]
 public class Resolution {
     string[] playerprefName = { "resolutionx", "resolutiony" };
     public SuperWideScreen SWC;
     public Text txtAspectRatioX;
     public Text txtAspectRatioY;
     public Text txtCurrentRes;
     public Button btnConfirm;

     Vector2 res;
     public Vector2 defaultRes;


     public void ApplyChanges(){
         //SWC.SetSize(res);
     }

     public void SavePlayerpref(){
         PlayerPrefs.SetFloat(playerprefName[0], res.x);
         PlayerPrefs.SetFloat(playerprefName[1], res.y);
     }

     public void ReadPlayerpref(){
         res.x = PlayerPrefs.GetFloat(playerprefName[0], defaultRes.x);
         res.y = PlayerPrefs.GetFloat(playerprefName[1], defaultRes.y);
     }
 }

 /*
  * 
  * 
  * 
  * 
  *  m  e  t  h  o  d  s
  * 
  * 
  * 
  */

 public Resolution resolutionControl;
 public Syphon syphonControl;
 public Hue hueControl;



 private void SaveStates() {
     
 }

 public void UpdateStates(){
     SaveStates();
 }

 private void ReadStates() {
     resolutionControl.ReadPlayerpref();
     syphonControl.ReadPlayerpref();
     hueControl.ReadPlayerpref();
 }

 private void setColors(){
     
 }

 //void Awake () {

 //}

 // Use this for initialization
 void Start () {
     resolutionControl = new Resolution();
     hueControl = new Hue();
     syphonControl = new Syphon();
     ReadStates();
     resolutionControl.ApplyChanges();
     syphonControl.ApplyChanges();
     hueControl.ApplyChanges();
 }
 
 // Update is called once per frame
 void Update () {
     
 }

}

I have linked the Button to the field Btn ,( see pic attached) but when I run the program, it shows me that it can't find btn.

NullReferenceException: Object reference not set to an instance of an object ControlPanel+BoolControl..ctor () (at Assets/ControlPanel/ControlPanel.cs:46)