主题：如何限制只对独立窗口进行操作，主窗口不能进行操作？

如何做到在一个主窗口中打开一个独立窗口时，限制只对独立窗口进行操作，主窗口不能进行操作？

出现错误：Object reference not set to an instance of an object.

主窗口和独立窗口要同时打开。

模式窗口与主窗口用同一数据表时，调用模式窗口时，主窗口的数据表出现空白，如何做到不出现这种情况？

If Tables("收购单价汇总").Rows.Count = 0 Then
    MessageBox.Show("请汇总后再打印!")
Else
    If Tables("收购初始号").Rows.Count = 0 Then
        Forms("输入初始号").Open()
        Dim tab As WinForm.TabControl = Forms("输入初始号").Controls("TabControl1")
        tab.SelectedIndex = 0
       
    Else
        Dim tt As Table = Tables("收购初始号")
        Dim i As Integer = tt.Rows.Count -1
        If tt.Rows(i)("收购开piao号") = tt.Rows(i)("收购凭证终止号") Then
            vars("kk") = 0
            Forms("输入初始号").Open()
            Dim tab As WinForm.TabControl = Forms("输入初始号").Controls("TabControl1")
            tab.SelectedIndex = 0
           
        Else
            If MessageBox.Show("是否打印?","确认",MessageBoxButtons.OKCancel,MessageBoxIcon.Question) =DialogResult.OK Then
                If tt.Rows(i)("收购开piao号") = "" Then
                    tt.Rows(i)("收购开piao号") = tt.Rows(i)("收购凭证起始号")
                Else
                    tt.Rows(i)("收购开piao号") = tt.Rows(i)("收购开piao号") + 1
                End If
                Dim idx As Integer
                idx = CInt(tt.Rows(i)("收购开piao号"))
                For Each t As Row In Tables("收购单价汇总")
                    t("打印") = True
                    t("收购凭证号") = format(idx,"000000000")
                Next
                For Each r As Row In Tables("打印收购窗口_Table1")
                    r("打印") = True
                    r("收购凭证号") = format(idx,"000000000")
                Next
                Dim td As Table =Tables("打印收购窗口_Table1")
                Dim ii As Integer = td.Rows.Count -1
                td.Rows(ii)("piao据张数") = 1
                Dim Book As New XLS.Book(ProjectPath & "Attachments\收购单.xls")
                Book.Marks.Add("用户名",User.Name)
                Dim fl As String = ProjectPath & "Reports\收购单.xls"
                Book.Build() '生成细节区
                Book.Save(fl) '保存工作簿
                Dim Proc As New Process '打开工作簿
                Proc.File = fl
                proc.verb = "print"
                Proc.Start()
                DataTables("收购单价汇总").DataRows.Clear
                DataTables("收购").LoadFilter = "收购凭证号 Is null And [重量] <> 0 And [日期] = #" & vars("adate") & "#"
                DataTables("收购").Load()
                Tables("收购").Sort = "编号"
                Dim cj As WinForm.ComboBox = e.Form.Controls("猪主姓名")
                cj.Text = ""
                cj.ComboList = DataTables("收购").GetComboListString("猪主姓名")
            End If
        End If
    End If
End If

上述代码中For Each r As Row In Tables("打印收购窗口_Table1")
                    r("打印") = True
                    r("收购凭证号") = format(idx,"000000000")
                Next
                Dim td As Table =Tables("打印收购窗口_Table1")
                Dim ii As Integer = td.Rows.Count -1
                td.Rows(ii)("piao据张数") = 1
不起作用。如何修改？

请老师指点一下！

十分希望老师能帮我一下！