Error - A control with the ID btnSave could not be found for the Trigger in Update Panel

Hi

   Below is the code

<asp:Content ID="Content1" ContentPlaceHolderID="MainContentPlaceHolder" runat="server"><asp:ScriptManager ID="ScriptManager1" runat="server"></asp:ScriptManager><asp:UpdatePanel runat="server" ID="updatePanel"><ContentTemplate><div class="row"><div class="col-md-12 col-sm-12 col-xs-12"><div class="x_title"><h5>Maintain Categories </h5></div><button type="button" class="btn btn-primary" data-toggle="modal" data-target="#myModal" onclick="clearTextBox();">Add New</button><br /><br /><asp:HiddenField ID="hfSelectedRecord" runat="server" /><asp:ListView ID="lvwCategories"  runat="server"><ItemTemplate><tr><td><%#Container.DataItemIndex+1 %></td><td><%#Eval("No") %></td><td><%#Eval("Description") %></td><td><asp:ImageButton ID="imgBtnEdit" CommandName="Edt" ToolTip="Edit a record." CommandArgument='<%#Eval("No") %>'
                                    runat="server" ImageUrl="~/Resources/Images/edit.png" /></td></tr></ItemTemplate></asp:ListView><div class="modal fade" id="myModal" tabindex="-1" role="dialog" aria-labelledby="myModalLabel" aria-hidden="true"><div class="modal-dialog"><div class="modal-content"><div class="modal-header"><button type="button" class="close" data-dismiss="modal" aria-hidden="true">&times;</button><h5 class="modal-title" id="H1">Add Category</h5></div><div class="modal-body"><form><div class="form-group"><label for="Code">Code</label><input type="text" class="form-control" id="txtNo" placeholder="Code"/></div><div class="form-group"><label for="Description">Description</label><input type="text" class="form-control" id="txtDescription" placeholder="Description"/></div></form></div><div class="modal-footer"><button type="button" class="btn btn-primary" id="btnSave" ">Save</button></div></div></div>       </div></div></div></ContentTemplate><Triggers><asp:PostBackTrigger ControlID="btnSave" /></Triggers></asp:UpdatePanel></asp:Content>

Thanks