create custom class

HI.

I want create custom class in my script and define custom variables in this class. for do this I create another public class and defined my variables inside it. but when i play the game, it give me below error:

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

This is my script:

 using System;
 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class _Player : MonoBehaviour {
 
     Player player;
 
     // Use this for initialization
     void Start ()
     {
 
     }
     
     // Update is called once per frame
     void Update () 
     {
         player.x = transform.position.x;
         player.y = transform.position.y;
     }
 
 }
 
 public class Player
 {
     public float x;
     public float y;
 }

Anyone know why unity give me this error?

Thanks.