Error : Object reference not set to an instance of an object body.Update

My unity is with the following problem:

NullReferenceException: Object reference not set to an instance of an object body.Update() (at Standard Assets/Assets/Menu/corpo.js: 20)

Tags and some elements as in Portuguese of Brazil

 #pragma strict
 var Vida: float;
 var TVida : TextMesh;
 
 var TempoQueda: float;
 
 function Start () {
 Vida = 100;
 TempoQueda=-1;
 }
 
 function Update () {
 
 if (TempoQueda>=0)
 {
 TempoQueda += Time.deltaTime;
 }
 
 TVida = transform.GetComponentInChildren(TextMesh);
 TVida.text = "Vida: "+Vida;
 
 
 
 }
 var testeInpacto : Vector3;
 function OnCollisionEnter(collision: Collision)
 {
   testeInpacto = collision.impactForceSum;
   if (testeInpacto.y<10)
   {
     if (collision.transform.tag!="agua")
     {
       if (TempoQueda>1)
       {
        Vida = Vida - (100 * (TempoQueda/3));
       }
     }
   }
   TempoQueda=-1; 
 }
 
 
 function OnCollisionExit(collisioninfo : Collision) {
 
 TempoQueda = 0 ;
 
 
 }