title: Troubles to open ajaxToolkit:ModalPopupExtender with JavaScript
body: <p>I'm trying to open a <code>ajaxToolkit:ModalPopupExtender</code> with <code>JavaScript</code> but when I run my code and I call the function from the code behind this crash and show this error.</p>
  
  <blockquote>
    <p>JavaScript runtime error: Unable to get property 'show' of undefined
    or null reference</p>
  </blockquote>
  
  <p>this is my <code>JavaScript</code>:</p>
  
  <pre><code>&lt;script&gt;
      function closeChangeArea() {
          $find('ModalChangeArea').hide();
      }
  
      function showChangeArea() {
          $find('ModalChangeArea').show();
      }
  &lt;/script&gt;
  </code></pre>
  
  <p>and this is my code:</p>
  
  <pre><code>protected void Btn_Click_Ch_Area(object sender, EventArgs e)
          {
              LinkButton lb = (LinkButton)sender;
              GridViewRow gr = (GridViewRow)lb.NamingContainer;
  
              Label ToolChange = (Label)gr.FindControl("Lbl_toolg");
              Txt_Tool_Reasign.Text = ToolChange.Text;
  
              ScriptManager.RegisterStartupScript(this, this.GetType(), "Pop", "showChangeArea();", true);
          }
  </code></pre>
  
  <p>this is my <code>ModalPoupExtender</code></p>
  
  <pre><code>&lt;ajaxToolkit:ModalPopupExtender
          ID="ModalChangeArea"
          runat="server"
          TargetControlID="hid"
          PopupControlID="ChangeArea"
          RepositionMode="RepositionOnWindowResizeAndScroll"
          DropShadow="true"
          PopupDragHandleControlID="moveArea"&gt;
      &lt;/ajaxToolkit:ModalPopupExtender&gt;
  </code></pre>