Door Missing

Hello, when I put two doors in one scene (one takes back to MainMenu, one takes player to SecondLevel) one of them goes missing. Really weird I've no idea why this is happening.

Any ideas? I can see them in "Scene" window of Unity, but when I play it & in game one door is always missing. But it "is" there. I can't see it, nor interact with it (which means it doesn't loadlevel or anything)

EDIT:

it says "MeshRenderer has been destroyed but you are still trying to access it" When I click to meshrenderer (untick) it really makes the door go invisible & missing. Now what do I have to do? Is it script based problem?

var levelToLoad : String;

private static var instance:Door;

public static function GetInstance() : Door {

return instance; }rer

function OnTriggerEnter(hit : Collider)

{

 Application.LoadLevel(levelToLoad);

}