Whats the difference between GetComponent and (Player)FindObjectOfType() ?

I am new to c# and game programming in general and while trying to learn about gizmos, i ran into a problem. My code for gizmos is as follows: void OnDrawGizmos() {

 GameObject go;
 Player pl = GetComponent<Player>();
 go = pl.gameObject;
     Gizmos.DrawWireCube(new Vector3(gameObject.transform.position.x,            gameObject.transform.position.y, gameObject.transform.position.z+2), 
                     new Vector3(1, 1, 1));
  
     Gizmos.DrawLine(gameObject.transform.position, go.transform.position);
     
 }

Now this is raising the following exception:

NullReferenceException: Object reference not set to an instance of an object MyGizmos.OnDrawGizmos () (at Assets/practise scripts/Gizmos/MyGizmos.cs:18) UnityEditor.DockArea:OnGUI()

i dont know where i'm going wrong...any help would be immensely appreciated. thank you