Bug in Object reference ?

so i have a code which has a public variable of my script type and i've assigned to it a game object through unity..everything works perfectly when i select my empty game object in the hierarchy , but once my selection is on something else i get the object reference error.. the error which says :-

NullReferenceException: Object reference not set to an instance of an object _Movement2D._disCharge (._Player player) (at Assets/Sample Assets/2D/Scripts/_Movement2D.cs:89) _Movement2D.Update () (at Assets/Sample Assets/2D/Scripts/_Movement2D.cs:75)

but what im not sure what happens there .. is it a bug? or it's my bad way of scripting things?

i'll post my code that has the error which is showing the error in unity... it's basic code for movement and such ..

 using UnityEngine;
 using System.Collections;
 
 public class _Movement2D : MonoBehaviour {
 
     private Rigidbody2D _refrenceToRigid2D;
     private Animator _refrenceToAnimator;
     private bool _facingRight = true;
     private bool _canJump;
     public float speed;
     public float jumpSpeed;
     public Transform startPos;
     public Transform endPos;
     public LayerMask groundLayer;
     /// <copied start>
     /// charging
     /// </copied end>
     public bool Charging = false;
     public _Player player;
 
     // Use this for initialization
     void Start () {
     
         _refrenceToRigid2D = this.rigidbody2D;
         _refrenceToAnimator = GetComponent<Animator>();
     }
     
     // Update is called once per frame
     void Update () {
     
         float move = Input.GetAxisRaw("Horizontal");
         _refrenceToAnimator.SetFloat("Speed",Mathf.Abs(move));
         _refrenceToRigid2D.velocity = new Vector2(move*speed , _refrenceToRigid2D.velocity.y);
 
         if (_facingRight == true && move < 0)
         {
 
             _facingRight = false;
             transform.rotation = Quaternion.Euler(transform.rotation.x, 180, transform.rotation.z);
         }
 
         else if (_facingRight == false && move > 0)
         {    
 
             _facingRight = true;
             transform.rotation = Quaternion.Euler(transform.rotation.x, 0, transform.rotation.z);
 
         }
 
 
         RaycastHit2D rayInfo = Physics2D.Linecast(startPos.position, endPos.position, groundLayer.value);
 
         if (rayInfo.collider != null)
         {
             _canJump = true;
             _refrenceToAnimator.SetBool("Jump",false);
         }
 
         if (Input.GetKey(KeyCode.Space)&& _canJump == true )
         {
             _refrenceToAnimator.SetBool("Jump",true);
             _refrenceToRigid2D.velocity = new Vector2(_refrenceToRigid2D.velocity.x,jumpSpeed);
             _canJump = false;
         }
 
         ////////////////////////////////////
         ////////////// Charge //////////////
         ////////////////////////////////////
         if (Input.GetButton("Fire1"))
         {
             _charging(player);
         }
         if (Input.GetButtonUp("Fire1"))
         {
             _disCharge(player);
         }
 
     }
 
     void _charging(_Player player)
     {
 
         player.playerArray[0].fCalculateCharge();
 
     }
 
     void _disCharge(_Player player)
     {
         player.playerArray[0].freleaseCharge();
     }
 
 
 }