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

so guys im trying to see how many protons are in nucleus and im trying to do this... this is the script of nucleus:

 using UnityEngine;
 using System.Collections;
 using UnityStandardAssets.CrossPlatformInput;
 public class NUCLEUS : MonoBehaviour {
     public float charge, mass, jump, tome, spd;
     Rigidbody2D myb;
     // Use this for initialization
     void Start () {
         myb = this.GetComponent<Rigidbody2D>();
 
     }
     public nums maynum = new nums (0, 0);
     public class nums{
         public int protnum, neutnum;
         public nums(int howmanyprotons, int howmanyneutrons){
         
             protnum += howmanyprotons;
             neutnum += howmanyneutrons;
 
 
         }
 
     }
     // Update is called once per frame
     void Update () {
         if (Input.GetKey("w")) {
             transform.Translate(Vector2.up * jump * tome*Time.deltaTime);
         }
         if (Input.GetKey("a")) {
             transform.Translate(Vector2.left * jump * tome*Time.deltaTime);
         }
         if (Input.GetKey("s")) {
             transform.Translate(Vector2.down * jump * tome*Time.deltaTime);
         }
         if (Input.GetKey("d")) {
             transform.Translate(Vector2.right * jump * tome*Time.deltaTime);
         }
         if (Input.GetKey(KeyCode.Space)) {
             print(maynum.protnum );
         }
         
         Vector2 move = new Vector2 (CrossPlatformInputManager.GetAxis ("Horizontal"), CrossPlatformInputManager.GetAxis ("Vertical")) * spd;
         
         myb.AddForce (move);
         
 
     }
 }

and this is the script of proton:

 using UnityEngine;
 using System.Collections;
 
 public class Proton : MonoBehaviour {
     
     public float charge, mass, spd;
     public Transform nucleus;
     private NUCLEUS nucl;
 
     // Use this for initialization
     void Awake ()
     {
         nucl = GetComponent<NUCLEUS>();
     }
     void Start () {
 
     }
     
     // Update is called once per frame
     void Update () {
         transform.position = Vector2.MoveTowards (transform.position, nucleus.transform.position, spd);
 
     }
 
     void OnTriggerEnter2D(Collider2D other){
         nucl.maynum.protnum++;
         print(nucl.maynum.protnum );
     }
     void OnTriggerExit2D(Collider2D other){
         nucl.maynum.protnum --;
         print(nucl.maynum.protnum );
     }
 
 
 
 }
 

as you can see it gives me NullReferenceException NullReferenceException: Object reference not set to an instance of an object Proton.OnTriggerEnter2D (UnityEngine.Collider2D other) (at Assets/Script/Proton.cs:26)

i dont know what to do... please help im stuck....