NullReferenceException : Object reference not set to an instance of an object

Im trying to access a variable from another script, but I get this error. What am I doing wrong?

 public var gravity : boolean;
 public var Force  : float;
 private var onTop : boolean;
 public var Player: Transform;
 private var PlayerScript : CharacterMotor = Player.GetComponent(CharacterMotor);
 function Update () {
     if(gravity){
         if(onTop){
             PlayerScript.Gravity -= Force;
         }    
     }
     onTop = false;
 }
 
 function OnTriggerStay(other : Collider){
     if(other.tag == "Player"){
         onTop = true;
     }
 }