Please help i have two issues my GridView becomes empty on enabling checkbox on form load and also GridView view returns error when i use the DropDownList code of on rowdatabound.

here is the error returned below.

System.NullReferenceException: 'Object reference not set to an instance of an object.'

System.Web.UI.WebControls.ListItemCollection.FindByValue(...) returned Nothing.

Public Class Newsearchdata
    Inherits System.Web.UI.Page
    Dim conString As String = ConfigurationManager.ConnectionStrings("STOREConnectionString1").ConnectionString

    Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        If Not IsPostBack Then
            Me.BindGrid()
        End If
    End Sub
    Private Sub BindGrid()
        Dim cmd As New SqlCommand("SELECT Name,class as Classr,class,stream, House, ACNOS,admno,sex,STDTYPE,Autofield FROM STUDENT")
        Stockbooks.DataSource = Me.ExecuteQuery(cmd, "SELECT")
        Stockbooks.DataBind()
        Stockbooks.UseAccessibleHeader = True
        Stockbooks.HeaderRow.TableSection = TableRowSection.TableHeader
    End Sub    

    Protected Sub OnRowDataBound(ByVal sender As Object, ByVal e As GridViewRowEventArgs)
        'If e.Row.RowType = DataControlRowType.DataRow AndAlso Stockbooks.EditIndex = e.Row.RowIndex Then

        ' If e.Row.RowType = DataControlRowType.DataRow AndAlso Stockbooks.EditIndex = e.Row.RowIndex Then
        If (e.Row.RowType = DataControlRowType.DataRow) Then
            Dim cell As TableCell = e.Row.Cells(1)
            Dim quantity As String = cell.Text
            'Find the DropDownList in the Row
            Dim ddlCountries As DropDownList = CType(e.Row.FindControl("ddlclasses"), DropDownList)
            ddlCountries.DataSource = GetData("SELECT DISTINCT(Class)as classr FROM Classes")
            ddlCountries.DataTextField = "Classr"
            ddlCountries.DataValueField = "Classr"
            ddlCountries.DataBind()

            'Add Default Item in the DropDownList
            ddlCountries.Items.Insert(0, New ListItem("Please select"))

            'Select the Country of Customer in DropDownList
            Dim country As String = CType(e.Row.FindControl("Lblclasses"), Label).Text
            If country = "" Then
                Exit Sub
            Else

                ddlCountries.Items.FindByValue(country).Selected = True
            End If
        End If
    End Sub
End Class