Unexplainable NullReferenceException?

So, making a tetris game, and trying to instantiate from a resources folder, here is the stack trace:

 namespace GameManager
 {
     public class GameManager:MonoBehaviour
     {
         Instantiator instance;
         void Start()
         {
             instance.Spawn();
         }
         public void CheckInput()
         {
             var posit = transform.position.y;
             if (posit != -10)
             {
                 if (Input.GetKeyUp(KeyCode.RightArrow))
                 {
                     transform.position += new Vector3(1, 0, 0);
                 }
                 else if (Input.GetKeyUp(KeyCode.LeftArrow))
                 {
                     transform.position += new Vector3(-1, 0, 0);
                 }
                 else if (Input.GetKeyUp(KeyCode.UpArrow))
                 {
                     transform.Rotate(0, 0, 90);
                 }
                 else if (Input.GetKeyUp(KeyCode.DownArrow))
                 {
                     transform.position += new Vector3(0, -1, 0);
                 }
             }
             else
             {
                 return;
             }
         }
         void Update()
         {
             CheckInput();
         }
     }
 }

Then,

 public interface Iinstantiatable
 {
     void SearchAndCall();
 }
 public class Instantiator:MonoBehaviour
 {
     Fire.Fire fireElement;
     Ice.Ice iceElement;
     Lightning.Lightning lightningElement;
     Terra.Terra terraElement;
     Water.Water waterElement;
     Wind.Wind windElement;
     public void Spawn()
     {
         System.Random RandomElement = new System.Random();
         int Case = RandomElement.Next(6);
         switch(Case)
         {
             case 1:
                 fireElement.SearchAndCall(); break;
             case 2:
                 iceElement.SearchAndCall(); break;
             case 3:
                 lightningElement.SearchAndCall(); break;
             case 4:
                 terraElement.SearchAndCall(); break;
             case 5:
                 waterElement.SearchAndCall(); break;
             case 6:
                 windElement.SearchAndCall(); break;
         }
     }
     void Start()
     {
         
     }
     void Update()
     {
 
     }
 }

Then, for example:

 namespace Fire
 {
     public class Fire:MonoBehaviour, Iinstantiatable
     {
         string TetroName;
         public void SearchAndCall()
         {
             GameObject Tetro = (GameObject)Instantiate(Resources.Load(ChooseFireShape(), typeof(GameObject)), new Vector2(0, 12f), Quaternion.identity);
         }
         public string ChooseFireShape()
         {
             System.Random RandomFire = new System.Random();
             int Case = RandomFire.Next(9);
             switch(Case)
             {
                 case 1:
                     TetroName = "FireTypes/I_Fire"; break;
                 case 2:
                     TetroName = "FireTypes/+_Fire"; break;
                 case 3:
                     TetroName = "FireTypes/Dot_Fire"; break;
                 case 4:
                     TetroName = "FireTypes/l_Fire"; break;
                 case 5:
                     TetroName = "FireTypes/LargeL_Fire"; break;
                 case 6:
                     TetroName = "FireTypes/Rect_Fire"; break;
                 case 7:
                     TetroName = "FireTypes/Sqr_Fire"; break;
                 case 8:
                     TetroName = "FireTypes/T_Fire"; break;
                 case 9:
                     TetroName = "FireTypes/U_Fire"; break;
             }
             return TetroName;
         }
         void Start()
         {
             
         }
         void Update()
         {
 
         }
     }
 }

The error occurs at GameManager.instance.Spawn().Following the trace back, I still don't see why there is a NullReferenceException. Any ideas why? Or maybe a bug in my code?