asp.net - 为什么我的 UpdatePanel 未在 ScriptManager 中注册时无法取消注册？

Question

我在下面的代码中使用模态弹出窗口。我收到以下错误：

Cannot unregister UpdatePanel with ID 'updatePnl' since it was not registered with the ScriptManager. This might occur if the UpdatePanel was removed from the control tree and later added again, which is not supported.
Parameter name: updatePanel

这可能是什么原因造成的？

这是我的代码：

<telerik:GridTemplateColumn HeaderStyle-Width="20%" HeaderText="See Your Assessment">
                    <ItemTemplate>
                        <asp:Button CommandName="Show" OnClick="grdMasterbtnClick" CommandArgument='<%#Eval("SurveyClientID")%>'
                            ID="btn" runat="server" Text="Your Assessment" />
                        <cc1:ModalPopupExtender CancelControlID="btnCancel" PopupControlID="modelPopUp" ID="ModalPopupExtender1"
                            runat="server" TargetControlID="imgClientFreeEval">
                        </cc1:ModalPopupExtender>
                        <asp:UpdatePanel ID="updatePnl" runat="server">
                            <ContentTemplate>
                                <asp:Panel ID="modelPopUp" runat="server" Visible="false" BackColor="AliceBlue">
                                    <p>
                                        These items will be permanently deleted and cannot be recovered. Are you sure?
                                    </p>
                                    <asp:Button ID="btnOk" Text="OK" runat="server" />
                                    <asp:Button ID="btnCancel" Text="Cancel" runat="server" />
                                </asp:Panel>
                            </ContentTemplate>
                        </asp:UpdatePanel>
                    </ItemTemplate>
                    <HeaderStyle Width="20%"></HeaderStyle>
                </telerik:GridTemplateColumn>