Script on 3d model gives me NullReferenceException

Hi, I've but using my script on a cube which served until now as a gun for my first person controller. The problem is, I've downloaded a 3d model, put it as the child of the cube, deactivated the script on the cube, and put it on my 3d gun model but now it doesn't work. On my cube it did. I get a NullReferenceException : Object reference not set to an instance of an object. But I see in the inspector my object (first person controller) is referenced to. I've tried to change the object referenced to but it doesn't work better. Thanks for your help!

public class RenderAspirateur : MonoBehaviour {

 public GameObject autreobjet;
 FPSMonScript autrescript;

 // Use this for initialization
 void Start () {

     transform.position = new Vector3 (-1000, -1000, -1000); 
     autrescript = autreobjet.GetComponent<FPSMonScript>();
 
 }
 
 // Update is called once per frame
 void Update () {
 
     if ((autrescript.aaspire == true)&&(autrescript.porteasp == true)) transform.position = new Vector3 (0, 0, 0);
     else transform.position = new Vector3 (-1000, -1000, -1000);
 

 }

}