Object reference not set to an instance of an object?

Code (csharp):

1. "NullReferenceException: Object reference not set to an instance of an object
2. TileScript.SetTileSprite (UnityEngine.Transform settilesprite) (at Assets/Resources/Scripts/TileScript.cs:139)
3. TileScript.Start () (at Assets/Resources/Scripts/TileScript.cs:91)"

I've been stuck on a problem for a while which seems really simple to fix though I've tried various different ways around it to no use. I've tried looking this up but cannot find anything similar and any help would be great. The 18th line is causing the error (so it must be something to do with the function?). I have the following code;

Code (csharp):

1. for (int y = 2; y < mapHeight-2; y++) {
2.       Transform tile = Instantiate (mountainTile, new Vector3 (Mathf.Round (y * tileSize), 2, 0), Quaternion.identity) as Transform;
3.       tile.parent = transform;
4.       map [2, y] = tile;
5.       SetTileSprite(tile);
6. }
7.  
8. Transform GetTileAt (int x, int y)
9.     {
10.         if (x < 0 || y < 0 || x > mapWidth || y > mapHeight)
11.         {
12.             Debug.LogWarning ("X or Y coordinate is out of bounds!");
13.             return null;
14.         }
15.         return map[x, y];
16.     }
17.  
18.     void SetTileSprite (Transform tile)
19.     {
20.         bool obj2right = false;
21.         int x = Mathf.FloorToInt (tile.transform.position.x);
22.         int y = Mathf.FloorToInt (tile.transform.position.y);
23.         if (GetTileAt (x + 1, y).GetComponent<BoxCollider2D> () != null) {
24.             obj2right = true;
25.             Debug.LogWarning ("obj2right true");
26.         }

 

Is the tile a prefab?

 

Yes

 

Try changing the tile to a GameObject, not a Transform.

 

That gives me

Code (csharp):

1. Assets/Resources/Scripts/TileScript.cs(91,25): error CS1503: Argument `#1' cannot convert `UnityEngine.Transform' expression to type `UnityEngine.GameObject'

(line 91 being line 2 in the bit I posted)

If I change line 2 to GameObject then I get a bunch of errors, is that the only way I can do it?