Checkbox check is not working

I have 4-5 checkbox inside gridview. I want to save its value into the database like,

If checked == true, then "Y" else "N"

I tried like below

foreach (GridRow row in GrdRights.Rows)
{
    xw.WriteElementString("ADD_FLAG", chkadd.Checked == true ? "Y" : "N");
    xw.WriteElementString("MODIFY_FLAG", chkedit.Checked == true ? "Y" : "N");
    xw.WriteElementString("VIEW_FLAG", chkview.Checked == true ? "Y" : "N");
    xw.WriteElementString("DEL_FLAG", chkdelete.Checked == true ? "Y" : "N");
}

but got error as

object reference not set to an instance of an object

I defined all checkboxes as below

GridDataControlFieldCell chkcellAdd = GrdRights.RowsInViewState[i].Cells[4] as GridDataControlFieldCell;
GridDataControlFieldCell chkcellView = GrdRights.RowsInViewState[i].Cells[5] as GridDataControlFieldCell;
GridDataControlFieldCell chkcellEdit= GrdRights.RowsInViewState[i].Cells[6] as GridDataControlFieldCell;
GridDataControlFieldCell chkcellDelete = GrdRights.RowsInViewState[i].Cells[7] as GridDataControlFieldCell;

CheckBox chkadd = chkcellAdd.FindControl("ChkIDAdd") as CheckBox;
CheckBox chkview = chkcellView.FindControl("ChkIDView") as CheckBox;
CheckBox chkedit = chkcellEdit.FindControl("ChkIDEdit") as CheckBox;
CheckBox chkdelete = chkcellDelete.FindControl("ChkIDDelete") as CheckBox;

I tried a link from here but getting the same error.

Please suggest what is wrong here

HTML of my gridview:-

<cc2:Grid ID="GrdRights" runat="server" FolderStyle="../Styles/Grid/style_12"
    AllowSorting="False" AutoGenerateColumns="False" AllowColumnResizing="true" AllowAddingRecords="false"
    AllowMultiRecordSelection="true" OnRowDataBound="GrdRights_RowDataBound" ViewStateMode="Enabled"
    PageSize="100">
    <ClientSideEvents OnClientSelect="FunMonthList" />
    <ScrollingSettings ScrollHeight="400px" />
    <Columns>
        <cc2:Column ID="Column1" DataField="MKEY" ShowHeader="false" HeaderText="Select" ReadOnly="true"
            Width="5%" runat="server">
            <TemplateSettings TemplateId="TemplateWithCheckbox" />
        </cc2:Column>

        <cc2:Column ID="Column2" DataField="parent_menu" HeaderText="MENU" Visible="true"
            ReadOnly="true" Width="10%" runat="server">
        </cc2:Column>
        <cc2:Column ID="Column4" DataField="child_menu_mkey" Visible="false" ReadOnly="true"
            Width="10%" runat="server">
        </cc2:Column>
        <cc2:Column ID="Column3" DataField="child_menu" HeaderText="SUB MENU" runat="server"
            Visible="true" Width="30%">
        </cc2:Column>

          <%--<cc2:Column ID="Column5" DataField="MKEY" ShowHeader="false" HeaderText="Select"                                                  ReadOnly="true" Width="5%" runat="server">
            <TemplateSettings TemplateId="TemplateWithCheckbox" />
        </cc2:Column>--%>

        <cc2:CheckBoxColumn Id="chkAdd" DataField="ADD_FLAG" ShowHeader="true" HeaderText="Add"
            Width="5%" runat="server" Align="right">
            <TemplateSettings TemplateId="TemplateWithChecAdd"  />
        </cc2:CheckBoxColumn>

        <cc2:CheckBoxColumn Id="chkEdit" DataField="MODIFY_FLAG" ShowHeader="true" HeaderText="Edit"
            Width="5%" runat="server" Align="right">
            <TemplateSettings TemplateId="TemplateWithChecEdit" />
        </cc2:CheckBoxColumn>

        <cc2:CheckBoxColumn Id="ChkView" DataField="VIEW_FLAG" ShowHeader="true" HeaderText="View"
            Width="6%" runat="server" Align="right">
            <TemplateSettings TemplateId="TemplateWithChecView" />
        </cc2:CheckBoxColumn>

        <cc2:CheckBoxColumn Id="ChkDelete" DataField="DEL_FLAG" ShowHeader="true" HeaderText="Delete"
            Width="8%" runat="server" Align="right"> 
            <TemplateSettings TemplateId="TemplateWithChecDelete" />
        </cc2:CheckBoxColumn>

    </Columns>
    <Templates>

        <cc2:GridTemplate ID="TemplateWithChecAdd">
            <Template>
                <asp:CheckBox runat="server" ID="ChkIDAdd" ToolTip="<%# Container.Value %>"  />
            </Template>
        </cc2:GridTemplate>

        <cc2:GridTemplate ID="TemplateWithChecView">
            <Template>
                <asp:CheckBox runat="server" ID="ChkIDView" ToolTip="<%# Container.Value %>" />
            </Template>
        </cc2:GridTemplate>

        <cc2:GridTemplate ID="TemplateWithChecEdit">
            <Template>
                <asp:CheckBox runat="server" ID="ChkIDEdit" ToolTip="<%# Container.Value %>" />
            </Template>
        </cc2:GridTemplate>

        <cc2:GridTemplate ID="TemplateWithChecDelete">
            <Template>
                <asp:CheckBox runat="server" ID="ChkIDDelete" ToolTip="<%# Container.Value %>" />
            </Template>
        </cc2:GridTemplate>

    </Templates>
</cc2:Grid>