Hat Tricks 2D game Game Controller Script Error

Error in renderer statement.
statement : float ballWidth = balls[0].renderer.bounds.extents.x;
full code:

Code (CSharp):

1. using UnityEngine;
2. using System.Collections;
3.  
4. public class HT_GameController : MonoBehaviour {
5.    
6.     public Camera cam;
7.     public GameObject[] balls;
8.     public float timeLeft;
9.     public GUIText timerText;
10.     public GameObject gameOverText;
11.     public GameObject restartButton;
12.     public GameObject splashScreen;
13.     public GameObject startButton;
14.     public HT_HatController hatController;
15.    
16.     private float maxWidth;
17.     private bool counting;
18.    
19.     // Use this for initialization
20.     void Start () {
21.         if (cam == null) {
22.             cam = Camera.main;
23.         }
24.         Vector3 upperCorner = new Vector3 (Screen.width, Screen.height, 0.0f);
25.         Vector3 targetWidth = cam.ScreenToWorldPoint (upperCorner);
26.         float ballWidth = balls[0].renderer.bounds.extents.x;
27.         maxWidth = targetWidth.x - ballWidth;
28.         timerText.text = "TIME LEFT:\n" + Mathf.RoundToInt (timeLeft);
29.     }
30.  
31.     void FixedUpdate () {
32.         if (counting) {
33.             timeLeft -= Time.deltaTime;
34.             if (timeLeft < 0) {
35.                 timeLeft = 0;
36.             }
37.             timerText.text = "TIME LEFT:\n" + Mathf.RoundToInt (timeLeft);
38.         }
39.     }
40.  
41.     public void StartGame () {
42.         splashScreen.SetActive (false);
43.         startButton.SetActive (false);
44.         hatController.ToggleControl (true);
45.         StartCoroutine (Spawn ());
46.     }
47.  
48.     public IEnumerator Spawn () {
49.         yield return new WaitForSeconds (2.0f);
50.         counting = true;
51.         while (timeLeft > 0) {
52.             GameObject ball = balls [Random.Range (0, balls.Length)];
53.             Vector3 spawnPosition = new Vector3 (
54.                 transform.position.x + Random.Range (-maxWidth, maxWidth),
55.                 transform.position.y,
56.                 0.0f
57.             );
58.             Quaternion spawnRotation = Quaternion.identity;
59.             Instantiate (ball, spawnPosition, spawnRotation);
60.             yield return new WaitForSeconds (Random.Range (1.0f, 2.0f));
61.         }
62.         yield return new WaitForSeconds (2.0f);
63.         gameOverText.SetActive (true);
64.         yield return new WaitForSeconds (2.0f);
65.         restartButton.SetActive (true);
66.     }
67. }
68.  

Moreover no public variables are seen in Inpector.Only this is seen
http://postimg.org/image/yltux0063/


same Goes for StartGame function
error:

 

The error message is telling you what is wrong & what to do to fix it for the renderer error.