How Do I Update a cSharp script var from js?

I've been looking into this a bit and still havn't got it to work. I'd be willing to either get them working together somehow, or use a shared global variable, or rewrite the csharp script to js (although it extends another csharp script i didnt write). I moved the csharp script to Standard Assets which seemed to help, but I still get an error: NullReferenceException: Object reference not set to an instance of an object BallControl.Update () (at Assets/Game/BallControl.js:37)

I'm using the following cSharp script to make a GUI for my game:

 using UnityEngine;
 using System.Collections;
 
 public class GameGui : VRGUI
 {
     public int lives = 5;
     public int score = 0;
     
     public override void OnVRGUI()
     {
         GUI.skin.label.fontSize = 30;
         GUILayout.BeginArea(new Rect(300, 100, 200, 100));
         GUILayout.Label("Lives: " + lives);
         GUILayout.EndArea();
         GUILayout.BeginArea(new Rect(900, 100, 200, 100));
         GUILayout.Label("Score: " + score);
         GUILayout.EndArea();
     }

     public void RemoveLife() {
     lives = lives - 1;    
 }
 }

This is the JS script i am trying to change the cSharp one from:

 #pragma strict
 var left = -100;
 var right = 100;
 var top = 100;
 var bottom = -100;
 var xV : float;
 var yV : float;
 var zV : float;
 var damage = 100;
 var guiscript : GameGui;  
 
 function Start () {
     xV = Random.Range(0,0.5);
     yV = Random.Range(0,0.5);
     zV = Random.Range(0,2.0);
 }
 
 function Update () {
     //move the ball
     transform.position.x = transform.position.x + xV;
     transform.position.y = transform.position.y + yV;
     transform.position.z = transform.position.z + zV;
     
     //ball bounces
     if(transform.position.x < left || transform.position.x > right) xV = -1 * xV;
     if(transform.position.y < left || transform.position.y > right) yV = -1 * yV;
     if(transform.position.z > right) zV = -1 * zV;
     
     //lose a life
     if(transform.position.z < left) {
         transform.position = Vector3(0,0,0);
         xV = Random.Range(0,0.5);
         yV = Random.Range(0,0.5);
         zV = Random.Range(0,2.0);
         
         guiscript = this.GetComponent("GameGui");  
         guiscript.RemoveLife();
     }
 }

I am just trying to change the GUI in csharp when stuff happens in js. Any help would be much appreciated!