Value cannot be null. Parameter name: control

        <cc1:ConfirmButtonExtender DisplayModalPopupID="ModalPopupExtender4" ID="ConfirmButtonExtender4"
            runat="server" TargetControlID="lnkremoveloc" Enabled="False" OnClientCancel="RemoveLocations">
        </cc1:ConfirmButtonExtender>
        <cc1:ModalPopupExtender ID="ModalPopupExtender4" OkControlID="btnDisable" CancelControlID="btnCancel"
            BackgroundCssClass="modalBackground" PopupControlID="pnlRemove" TargetControlID="lnkremoveloc"
            runat="server">
        </cc1:ModalPopupExtender>
        <asp:Panel CssClass="modalPopup" ID="PnlRemove" runat="server">
            <!-- Common Popup Control Begin -->
            <table class="tblCommonPopup" width="400" cellpadding="0" cellspacing="0">
                <tr>
                    <td class="topLeft">
                    </td>
                    <td class="topMiddle">
                    </td>
                    <td class= "topRight">
                    </td>
                </tr>
                <tr>
                    <td colspan="5" class="middle" align="center">
                        <!-- Content Area Begin -->
                        <table>
                            <tr>
                                <td colspan="1" style="padding-top: 10px;" title="Select">


                                </td>
                                <td colspan="4" style="padding-top: 10px;" title="Location Name">

                                </td>
                            </tr>
                           开发者_开发问答 <tr>
                                <td align="center" colspan="5">
                                    <input id="btnDisable" type="button" value="Disable" class="popupButton" />
                                    <input id="Cancel" type="button" value="Cancel" class="popupButton" />
                                </td>
                            </tr>
                        </table>
                        <!-- Content Area Ends -->
                    </td>
                </tr>
                <tr>
                    <td class="bottomLeft">
                    </td>
                    <td class="bottomMiddle">
                    </td>
                    <td class="bottomRight">
                    </td>
                </tr>
            </table>
            <!-- Common Popup Control End -->
        </asp:Panel>

That error means that there's a Dependency Property that has a null value where one is not allowed. Unfortunately you only pasted partial code so I can't tell which property it is - please paste the full code of your control.