title: panel hide and show in NGUI
body: <p>I am new to <code>NGUI</code> and <code>unity 3d</code>.
    I have two panels in a <code>ui</code> root. its named as <code>firstPanel</code> and <code>secondPanel</code>. secondPanel is deactivated in scene. In <code>firstPanel</code> i have so many buttons and one is a <code>play</code> button, that is image button. While Clicking on <code>play</code> button, <code>firstPanel</code> should get <strong>hide</strong> and <code>secondPanel</code> should <strong>show</strong>.I adde a new Script to <code>play</code> button and written code in it</p>
  
  <pre><code>void OnClick(){
      GameObject panel2  = GameObject.Find("secondPanel");
      NGUITools.SetActive(panel2,true);       
      GameObject panel1  = GameObject.Find("firstPanel");         
      NGUITools.SetActive(panel1,false);
  
  }
  </code></pre>
  
  <p>But I  get this <strong>Error</strong> : <code>"NullReferenceException"</code>
   In which script of <code>ngui</code> i have to edit and how can i do it? please help me to solve this issue
      Thanks in advance.</p>