ปัญหาคำสั่งUpdate Object reference not set to an instance of an object.

Imports System.Data
Imports System.Data.SqlClient
Partial Class Purchase_Default
    Inherits System.Web.UI.Page

    Dim objConn As SqlConnection
    Dim objCmd As SqlCommand
    Dim strSQL As String
    Dim CusID As String = ""



    End Sub
    Protected Sub gvManageBindData()

        strSQL = "SELECT * FROM CUS"
        Dim dtReader As SqlDataReader
        objCmd = New SqlCommand(strSQL, objConn)
        dtReader = objCmd.ExecuteReader()

        gvManage.DataSource = dtReader
        gvManage.DataBind()

        dtReader.Close()
        dtReader = Nothing

    End Sub
    Protected Sub dvEditBindData()

        strSQL = "SELECT * FROM  Cus where CUSID = '" & CUSID & "' "
        Dim dtReader As SqlDataReader
        objCmd = New SqlCommand(strSQL, objConn)
        dtReader = objCmd.ExecuteReader()

        dvEdit.DataSource = dtReader
        dvEdit.DataBind()

        dtReader.Close()
        dtReader = Nothing

    End Sub
    Protected Sub Page_UnLoad()
        objConn.Close()
        objConn = Nothing
    End Sub
    
  
End Class

        strSQL = "Update [Book] SET [BookISBN] = @txtISBN, "
        strSQL &= "[BookName] = @txtName, "
        strSQL &= "[BookDesc] = @txtDesc, "
        strSQL &= "[BookPrice] = @txtPrice, "
        strSQL &= "[BookQuantity] = @txtQuantity "
        strSQL &= "WHERE [BookID] = @BookID"
 
        objCmd = New SqlCommand(strSQL, objConn)
        objCmd.Parameters.Add("@txtISBN", SqlDbType.NVarChar)
        objCmd.Parameters.Add("@txtName", SqlDbType.NVarChar)
        objCmd.Parameters.Add("@txtDesc", SqlDbType.NVarChar)
        objCmd.Parameters.Add("@txtPrice", SqlDbType.NVarChar)
        objCmd.Parameters.Add("@txtQuantity", SqlDbType.NVarChar)
        objCmd.Parameters.Add("@BookID", SqlDbType.NVarChar)
        objCmd.Parameters("@txtISBN").Value = txtISBN.Text
        objCmd.Parameters("@txtName").Value = txtName.Text
        objCmd.Parameters("@txtDesc").Value = txtDesc.Text
        objCmd.Parameters("@txtPrice").Value = txtPrice.Text
        objCmd.Parameters("@txtQuantity").Value = txtQuantity.Text
        objCmd.Parameters("@BookID").Value = BookID

        objCmd.ExecuteNonQuery()

Dim txtQuantity As TextBox = CType(dvEdit.FindControl("txtBookQuantity"), TextBox)

Protected Sub gvManage_RowDeleting(ByVal sender As Object, ByVal e As System.Web.UI.WebControls.GridViewDeleteEventArgs) Handles gvManage.RowDeleting
        strSQL = "DELETE FROM Book WHERE BookID = '" & gvManage.DataKeys.Item(e.RowIndex).Value & "'"
        objCmd = New SqlCommand(strSQL, objConn)
        objCmd.ExecuteNonQuery()

        gvManageBindData()

    End Sub