facing error with my panel visible

Hi 
i am facing an error with the behind code as i click on likebtn the Visual studio show message: "  NullReference not set to an instance of an object. Object reference not set to an instance of an object. "
<div class="adsdetlbl"> <asp:Panel ID="notipanel" runat="server" Visible="false"> <div class="alert-box warning"> <asp:Label ID="warnotiflbl" runat="server"></asp:Label></div> </asp:Panel> <asp:Panel ID="suscpanel" runat="server" Visible="false"> <div class="alert-box success"> <asp:Label ID="sucsnotiflbl" runat="server"></asp:Label> ...