主题：关于窗口dropdownclosed事件代码的疑问

我在窗口的dropdownclosed中设置了如下的代码：

If e.Selected Then           '如果选择了值
    Dim tbl As Table = Tables("rkdrop_Table1")
    Dim bh As String = tbl.Current("hh")               '将要做订单的商品的货号
    Dim sdr As DataRow = DataTables("sp").Find("hh = '" & bh & "'")                '该商品在sp表中的对应记录
    Dim fws As String = sdr("jyfwsx")                'sp对应记录的jyfwsx
    Dim fwq As String = DataTables("bqyzz").DataRows(0)("jyfwlb")               '本企业的经营范围
    Dim gdr As DataRow = Tables("cgdmxb").Current.DataRow. GetParentRow("cgddb")               '采购订单明细表的对应父行
    Dim ghm As String = gdr("ghs")                    'cgddb中的对应行的ghs列
    Dim hdr As DataRow = DataTables("ghszz").Find("ghsmc = '" & ghm & "'")
    Dim fwg As String = hdr("jyfwlb")
    If tbl.Current IsNot Nothing Then
        If tbl.Current("kz") = True Then
            MessageBox.Show("该商品批准证明文件资质已过期,暂停采购计划!","提示", MessageBoxButtons.Ok, MessageBoxIcon.warning)
            e.Form.DropDownBox.Value = ""
            Tables("cgddb.cgdmxb").Current("hh") = ""
            Tables("cgddb.cgdmxb").Current("pm") = ""
            Tables("cgddb.cgdmxb").Current("sccj") = ""
            Tables("cgddb.cgdmxb").Current("gg") = ""
            Tables("cgddb.cgdmxb").Current("dw") = ""
        ElseIf fwq.contains( fws ) = False OrElse fwg.contains(fws) = False Then
            MessageBox.show("购销双方或单方不具备经营该商品的范围条件!")
            e.Form.DropDownBox.Value = ""
            Tables("cgddb.cgdmxb").Current("hh") = ""
            Tables("cgddb.cgdmxb").Current("pm") = ""
            Tables("cgddb.cgdmxb").Current("sccj") = ""
            Tables("cgddb.cgdmxb").Current("gg") = ""
            Tables("cgddb.cgdmxb").Current("dw") = ""
        Else
            e.Form.DropDownBox.Value = tbl.Current("pmzjm")
            Tables("cgddb.cgdmxb").Current("hh") = tbl.Current("hh")
            Tables("cgddb.cgdmxb").Current("pm") = tbl.Current("pm")
            Tables("cgddb.cgdmxb").Current("sccj") = tbl.Current("sccj")
            Tables("cgddb.cgdmxb").Current("gg") = tbl.Current("gg")
            Tables("cgddb.cgdmxb").Current("dw") = tbl.Current("dw")
        End If
        e.Form.DropDownBox.CloseDropDown()
    End If
End If

前边试验时是好用的，但是这两天不知道怎么回事，提示错误：

                                                    未将对象引用设置到对象的实例。

求解答！