Retrieve checkbox with char value

There are 2 checkbox and when the database record is User1, User2 both the checkbox will be checked, else if one value, only one checkbox is checked.
Have problem retrieving the checkbox value after i click on edit button on Gridview: 
I got the error that object is not set to reference -GridView:

<EditItemTemplate>
                            <asp:CheckBoxList ID="ch_Users2" runat="server" EnableViewState="false"></asp:CheckBoxList>  
                                </EditItemTemplate>               &nb ...