Change variable of ex2D Sprite using the component ExScreenPosition via code .js script

Hi,

I am simply trying to access and set a variable of a Component (script) added to prefab which is Instantiated and stored in a array.

     #pragma strict
 
 //An array to store all the Coordinates of Game Tiles for that column
 public var GameGridPositions: Vector2[,];
 
 //X Pixel position of first tile (topLeft)
 var FirstTilePosX         :int = 14;
 
 //Y Pixel position of first tile (topLeft)
 var FirstTilePosY         :int = 85;
 
 //Width of tile to calculate offset for each position
 var GameTileWidth         :int = 84;
 
 //Number of Rows to know how many position to calculate for
 var NumberOfRows        :int = 9;
 
 //Number of Columns to know how many position to calculate for
 var NumberOfColumns        :int = 7;
 
 //How many unique game items to make
 var MinGameItemCount    :int = 7;
 var MaxGameItemCount    :int = 14;
 var GameItemPrefab : exSpriteAnimation ;
 
 function Awake () {
     
     //Initialise a Array with +1 to make first points 1x1 instead of 0x0    
     GameGridPositions = new Vector2[NumberOfColumns+1,NumberOfRows+1]; 
     
     //Populates the array starting from 1 (not 0) 
     for(var r = 1; r <= NumberOfRows; r++)
         {
         for (var c = 1; c <= NumberOfColumns; c++) {
              GameGridPositions [c,r] = Vector2 (FirstTilePosX + (GameTileWidth * c) ,FirstTilePosY + (GameTileWidth * r));
              //Debug.Log("C:" + c + " | R:" + r + " = " + GameGridPositions[c,r]);
              
              } 
          }
     
     //How many GameItems are made
     var GameItemCount = Random.Range(MinGameItemCount, MaxGameItemCount); 
     Debug.Log(GameItemCount);
     
     //Makes array to hold all game items
     var GameItems : exSpriteAnimation[] = new exSpriteAnimation[GameItemCount];
 
     //Populates the array with required number of GameItems
     for(var i = 0; i <= GameItemCount; i++) {
         
         var GameItem  : exSpriteAnimation  ;
         GameItem = Instantiate(GameItemPrefab, GameGridPositions[0,2] , Quaternion.identity) as exSpriteAnimation;
         GameItems[i] = GameItem;
         GameItems[i].name = "GameItem" + i;
 
         //GameItems[i] = Instantiate(GameItem, GameGridPositions[0,2] , Quaternion.identity) as exSpriteAnimation;
          
          } 
              
     GameItems[2].GetComponent("ExScreenPosition").ScreenX = 100;
 
 
 }

Have also tried; GameItems[2].GetComponent("ExScreenPosition"); GameItems[2].ScreenX = 100;

I get this error : BCE0019: 'ScreenX' is not a member of 'UnityEngine.Component'.

Also unrealted i get this error NullReferenceException: Object reference not set to an instance of an object. exSpriteAnimState..ctor.

I just want to be able to use the array of co-ordinates I have with the sprites to position them. Also going to use HOTween to make it smooth transitions.

Thank you in advance for anyone willing and able to help :)