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

Hi guys im new to Unity and i've got this error which i wanted to basically Press "E" to display the GUI.DrawTexture. However i got this error which i dont understand where the problem is. Here's my code.

 var guiShow : boolean = false;
 
 var Note : Texture2D;
 
 
 function OnTriggerStay ()
 
 {
        if (Input.GetKey(KeyCode.E))
          {
            guiShow = true;
            OnGUI();
          }
 
 }
 
 function OnTriggerExit ()
 {      
     guiShow = false;
 }
 
 function OnGUI() 
 
 {    
     if (guiShow == true) 
   
       {
     
        GUI.DrawTexture(Rect(Screen.width/20, Screen.height/20,1024,512) , Note); //<-something wrong
 
       }
 }