unable to pass parameters to a C# script (no GameObject)

Alright, hi all. I'm pretty new to Unity and I think I'm missing something... The problem is that I cannot access to a method on another script (which isn't attached to any gameObject) because everytime i try to, while playing the simulation, the "NullRefereceException" error appears...

What I'm trying to do is to cast a spell: a fireball prefab (just some particle effects) put in the Resources directory, has to be instantiated and then it should move to the target position.. the caster is the player which has the "CharacterSpellCast" script attached, the target is the gameObject i click.

Here there are the codes:

Casting spells script

 public class CharacterSpellCast : MonoBehaviour {
 
     public GameObject Player;
     private RaycastHit hit;
     private Transform target;
     Fireball fireball;
     
 
     void Start () {
         fireball = FindObjectOfType(typeof(Fireball)) as Fireball; 
         Player=this.gameObject;
 
     }
 
     void Update () {
         if(Input.GetMouseButtonDown(0)){
             Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);
             if(Physics.Raycast(ray,out hit)&& hit.transform.gameObject!=this.gameObject){
                 target=hit.transform;
                 print (target.name);
                 fireball.Cast(target,Player.transform);
             }
         }
     }
 
 }

Fireball Script

 public class Fireball : MonoBehaviour {
 
     private Vector3 velocity = Vector3.forward;
 
     public void Cast (Transform Target,Transform Caster) {
         print("into");
         //GameObject fireball=GameObject.Find("Fireball");
         GameObject fireball = Resources.Load("Fireball") as GameObject;
 
         float time=0.5f*Time.deltaTime;
         if(fireball!=null){
             GameObject Spell = Instantiate(fireball);
             print ("into if");
             Spell.transform.position= Vector3.SmoothDamp(Caster.position,Target.position,ref velocity,time);
         }
         else 
             print("null");
     }
 }

The problem seems to be on fireball.Cast(target,Player.transform); but I'm not figuring how to deal with it... thanks for the help.