how To disable textbox inside gridview

Question

How to disable textboxes inside gridview Edititem section based on some condition below is the code which says object reference is not set 

 Private Sub grdUserReport_RowDataBound(sender As Object, e As System.Web.UI.WebControls.GridViewRowEventArgs) Handles grdUserReport.RowDataBound

        For Each row As GridViewRow In grdUserReport.Rows

            If row.RowType = DataControlRowType.DataRow Then

                For i As Integer = 1 To row.Cells.Count - 1

                    If row.Cells(i).Controls.OfType(Of TextBox)().ToList().Count > 0 Then

                        ' row.Cells(i).Controls.OfType(Of TextBox)().FirstOrDefault().Visible = isChecked

                        row.Cells(i).Controls.OfType(Of TextBox)().FirstOrDefault().Enabled = True

                    End If

                    Dim country As String = TryCast(row.Cells(0).FindControl("litType"), Label).Text

                    Dim strColumn As String = DataBinder.Eval(e.Row.DataItem, "ControlScr").ToString

                    ' Dim check As String = TryCast(row.Cells(1).FindControl("litStatus"), Label).Text

                    ' Dim txtForboy As String = TryCast(row.Cells(0).FindControl("QTotal"), Literal).Text

                    Dim txtBoxMen As TextBox = TryCast(grdUserReport.HeaderRow.FindControl("QTotal"), TextBox)

                    If country.Trim() = "DQTwoValueInteger.ascx" Then

                        txtBoxMen.Enabled = False

                        row.Cells(1).Controls.OfType(Of TextBox)().FirstOrDefault().Enabled = False

                        row.Cells(9).Controls.OfType(Of TextBox)().FirstOrDefault().Enabled = False

                        row.Cells(10).Controls.OfType(Of TextBox)().FirstOrDefault().Enabled = False

                        row.Cells(6).Controls.OfType(Of TextBox)().FirstOrDefault().Enabled = False

                    End If

                Next

            End If

        Next

    End Sub