Problem in changing texture using javascript

I am changing signal texture color in unity as-

 public class signal_change : MonoBehaviour {
 
         int Counter=0;
         int LastCounter=0;
 
         void Start () 
         {
 
         }
         void Awake()
         {
             //rend = gameObject.GetComponent(Renderer);
         }
         void Update () 
         {
             if (Input.GetKeyDown(KeyCode.U))
             {
                 MyFunction("");
             }  
 
             if (LastCounter != Counter) {
 
             Counter=LastCounter;
 
                     Texture txt=null;
             switch(Counter)
                 {
                 case 1:txt=Resources.Load("caution-signal", typeof(Texture2D)) as Texture;break;
                 case 2:txt=Resources.Load("attention-signal", typeof(Texture2D)) as Texture;break;
                 case 3:txt=Resources.Load("proceed-signal", typeof(Texture2D)) as Texture;break;
                 case 4:txt=Resources.Load("stop-signal", typeof(Texture2D)) as Texture;break;
                 }
         
                 renderer.material.mainTexture =  txt;
                                 }
         
     }
 
         public void MyFunction(string data)
         {
             LastCounter++;
             if(LastCounter>4)
                 LastCounter=1;
             //rend.material.mainTexture =  txt;
         Application.ExternalCall( "SayHello", LastCounter.ToString() );
         }
 
     }

its working fine but i want to change signal texture on the button click using javascript its not working properly.

my javascript call is-

 var u = new UnityObject2();
 u.getUnity().SendMessage("/MyTrafficLight/ID189", "MyFunction", "");