ASPxCheckBox in gridView keeps returning a null value

I have a gridview that contains checkboxes. The problem I am having is that when I try to access my checkbox in my C# code it keeps returning a null value. Its almost as if C# does not recognise the ID value of the checkbox in the aspx.

My GridView

<dx:ASPxGridView ID="grvQualificationScheduleDetails" runat="server" KeyFieldName="ModuleEnrollmentId" SettingsPager-Mode="ShowAllRecords">
                                <Columns>
                                    <dx:GridViewDataColumn FieldName="Academicyear" VisibleIndex="0" Caption="Year" />
                                    <dx:GridViewDataColumn FieldName="ModuleName" VisibleIndex="1" Caption="Module Name" />
                                    <dx:GridViewDataColumn FieldName="ScheduleName" VisibleIndex="2" Caption="Schedule Name" />
                                    <dx:GridViewDataColumn FieldName="ScheduleGroupName" VisibleIndex="3" Caption="Group Name" />
                                    <dx:GridViewDataColumn FieldName="Result" VisibleIndex="4" Caption="Status" />
                                    <dx:GridViewDataColumn FieldName="ModuleEnrollmentId" VisibleIndex="5" Visible="false" Caption="ModuleEnrollmentId" />
                                    <dx:GridViewDataColumn FieldName="" VisibleIndex="6">
                                    <DataItemTemplate>
                                     <dx:ASPxCheckBox ID="ChkSelected" OnCheckedChanged="ChkSelected_CheckedChanged" Checked='<%#((IsSelected((int)Eval("ModuleEnrollmentId"))==true))?false:true %>' AutoPostBack="true" runat="server"></dx:ASPxCheckBox>
                                   </DataItemTemplate>
                                   </dx:GridViewDataColumn>
                                  
                                </Columns>
                            </dx:ASPxGridView>

My C# method

 protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {

        ASPxCheckBox chk = (ASPxCheckBox)grvQualificationScheduleDetails.FindControl("ChkSelected");
        chk.Checked = false; 
    }
}