Enable disable CheckBox in GridView based on column value on RowEditing Event using C# in ASP.Net

Hello,

I get the error message "Object reference not set to an instance of an object." when I try to enable a checkbox in the gridview. Here's my code: 

        protected void gv_RowEditing(object sender, GridViewEditEventArgs e)
        {
            int index = Convert.ToInt32(e.NewEditIndex);
            Label result = (Label)gv.Rows[index].FindControl("lblResultAuto");

            gv.EditIndex = e.NewEditIndex;

            if (result.Text == "N/A")
            {
                CheckBox cbYes = (CheckBox)gvChecks.Rows[index].FindControl("cbResultManualYes");
                cbYes.Enabled = true;

                CheckBox cbNo = (CheckBox)gvChecks.Rows[index].FindControl("cbResultManualno");
                cbNo.Enabled = true;
            }

            bindChecks();
        }

 Any kind of help will be appreciated.

Thank you in advance.

<asp:GridView ID="gv" runat="server" AutoGenerateColumns="false"   
    OnRowCancelingEdit="gv_RowCancelingEdit"   
    OnRowEditing="gv_RowEditing"   
    OnRowUpdating="gv_RowUpdating"  CssClass="mTable">  
    <Columns>  
        <asp:TemplateField HeaderText="Check" ItemStyle-Width="25">  
        <ItemTemplate>  
             <asp:Label ID="lblResultAuto" runat="server" Text='<%#Eval("CheckRes") %>' Width="25"></asp:Label>                
         </ItemTemplate>               
    </asp:TemplateField>  
            <asp:TemplateField HeaderText="Valid" ItemStyle-Width="25">  
        <ItemTemplate>  
         <asp:CheckBox ID="lblResultManual" runat="server" Enabled="False" />            
        </ItemTemplate>  
        <EditItemTemplate>  
             <asp:CheckBox ID="cbResultManualYes" runat="server" Enabled="false"/>    
        </EditItemTemplate>  
    </asp:TemplateField>  
    <asp:TemplateField HeaderText="Not Valid" ItemStyle-Width="25">  
        <ItemTemplate>  
         <asp:CheckBox ID="lblResultManual1" runat="server" Enabled="False" />            
        </ItemTemplate>  
        <EditItemTemplate>  
             <asp:CheckBox ID="cbResultManualΝο" runat="server" Enabled="false"/>    
        </EditItemTemplate>  
    </asp:TemplateField>  
         <asp:TemplateField >  
            <ItemTemplate>  
                <asp:LinkButton ID="btnEdit" Text="Edit" runat="server" CommandName="Edit" />                        
            </ItemTemplate>  
            <EditItemTemplate>  
                <asp:LinkButton ID="btnUpdate" Text="Update" runat="server" CommandName="Update" />  
                <asp:LinkButton ID="btnCancel" Text="Cancel" runat="server" CommandName="Cancel" />  
            </EditItemTemplate>  
        </asp:TemplateField>  
    </Columns>  
</asp:GridView>