PopUp div is not displaying by using Javascript function (JavaScript runtime error: Unable to get property 'style' of undefined or null reference)

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ASP.NET

Javascript

PopUp div is not displaying by using Javascript function
------------------------------------------------------------
Js function
-------------------

function btnExportLE_click() {
        debugger;
        document.getElementById('<%=dvladen.ClientID %>').style.display = "block";
        document.getElementById('<%=dvHider.ClientID %>').style.display = "block";
        return false;
    }

Style
-----

.dvladen
 {
     z-index: 2;
     position: absolute;
     width: 599px;
     height: 398px;
     border-width: 0px;
     top: 280px;
     left: 573px;
     background-color: White;
}

DIV
--------

<div id="dvladen" runat="server" class="popup" visible="false" >
<p> HAIII</p>
</div>
<div id="dvHider" runat="server" class="hider">
</div>

Then I called Js function in Button Click

<asp:Button ID="btnExportLE" runat="server" class="btnNew" Text="Export LE"
OnClientClick="return btnExportLE_click();" />

PAge Load
----------
dvladen.Style["display"] = "none";


But It Showing An error like

JavaScript runtime error: Unable to get property 'style' of undefined or null reference


please help me to solve this..


Regards
Jithesh