Relace panel by this
<asp:Panel ID="pnlPanelDemo" runat="server" CssClass="modalPopup" ViewStateMode="Disabled">
<asp:TextBox ID="txtComment" runat="server" /><br />
<asp:RequiredFieldValidator ID="RequiredFieldValidator1" ControlToValidate="txtComment"
runat="server" ForeColor="Red" ErrorMessage="This is Required" ValidationGroup="Save" Display="Dynamic"></asp:RequiredFieldValidator><br />
<cc1:ValidatorCalloutExtender ID="RequiredFieldValidator1_ValidatorCalloutExtender"
BehaviorID="RequiredFieldValidator1_ValidatorCalloutExtender" runat="server"
Enabled="true" TargetControlID="RequiredFieldValidator1" PopupPosition="Right">
</cc1:ValidatorCalloutExtender>
<asp:Button ID="btnSave" Text="Save" runat="server" ValidationGroup="Save" />
<asp:Button ID="btnCancel" runat="server" ValidationGroup="Save" Text="Cancel" OnClientClick="return Cancel()" />
</asp:Panel>
