Instantiate error on Windows but works in the designer execution

Hello,

I have a strange error in my program with instantiate. I have two objects in my designer :

1. Word ==> initialize all letters for the word

2. LetterWord ==> letter show on the screen with it's own behaviour

When i run my program with ctrl+P in designer, it works correctly and i don't have any issue. When i run my program on Windows (.exe), i have an exception :

 ArgumentException: The thing you want to instantiate is null.
   at UnityEngine.Object.CheckNullArgument (System.Object arg, System.String message) [0x00006] in C:\buildslave\unity\build\Runtime\Export\UnityEngineObject.cs:102 
   at UnityEngine.Object.Instantiate (UnityEngine.Object original, Vector3 position, Quaternion rotation) [0x00000] in C:\buildslave\unity\build\Runtime\Export\UnityEngineObject.cs:79 
   at Word.Start () [0x0006d] in C:\Users\blahblah\Documents\Unity\JumperOne\Assets\Scripts\Word.cs:32 

On my designer, LetterWord is an empty object with a LetterWord.cs attached and a Word empty object with a Word.cs attached.

Word.cs use this in the Start() method and an object (LetterWord) drag on his :

 >         string LetterName = "iWordObject";
 >         letterHolder = new
 > GameObject(LetterName).transform;
 >         int _index = 0;         // show word to
 > find below grid         foreach (int
 > _letters in localWord) {             Vector3 letterPosition = new Vector3 (_x, _y,
 > 0);             localWordTile =
 > Instantiate(wordTile, letterPosition, 
 > Quaternion.identity) as GameObject;
 >             localWordTile.transform.position =
 > letterPosition;
 >             localWordTile.transform.parent =
 > letterHolder;             localWordTile.tag =
 > "Letters";
 >             localWordTile.GetComponent<LetterWord>
 > ().setLetterValue (_letters - 65);
 >             localWordTile.GetComponent<LetterWord>
 > ().setLetterIndex (_index);             _x = _x
 > - 1;             _index = _index + 1;         }

WordTile and localWordTile are GameObject.

LetterWord object use a LetterWord.cs script with some behaviour.

Two of us herited of MonoBehavious method.

What is the issue ? Why it can work in the designer and crash on Windows player ...

Thanks for your help.