Ms access 如何识别对象是否持有空值

我在引用基础表的表单文本框上有一个控件。我希望有一个命令，在单击时将空值更新为未启动的值。单击后，我得到一条运行时错误消息424-Object-needed

Private Sub Comando50_Click()

  If [txtScope] = ("not started") Then
      [txtScope] = ("proposed")

  ElseIf [txtScope] = ("proposed") Then
      [txtScope] = ("accepted")

  ElseIf [txtScope] Is Null Then
      [txtScope] = ("not started")


  ElseIf [txtScope] = ("accepted") Then
      [txtScope] = ("rejected")


  ElseIf [txtScope] = ("rejected") Then
       [txtScope] = ("on track")

  ElseIf [txtScope] = ("on track") Then
      [txtScope] = ("needs attention")


  ElseIf [txtScope] = ("needs attention") Then
      [txtScope] = ("critical")

  ElseIf [txtScope] = ("critical") Then
      [txtScope] = ("complete")

  ElseIf [txtScope] = ("complete") Then
      [txtScope] = ("not started")

  End If


End Sub

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假设txtScope是表单上的文本框或字段，那么您应该这样引用它：

Me.txtScope

或

我！txtScope

不需要在文本值周围加括号：

Private Sub Comando50_Click()

  If Me.txtScope = "not started" Then
      Me.txtScope = "proposed"    
  ElseIf Me.txtScope = "proposed" Then
      Me.txtScope = "accepted"
'etc etc
End Sub

---

然而，它仍然没有获取空值。这可能是因为对象实际上来自下拉菜单吗？ElseIf Me.txtScope=Null，则Me.txtScope=未启动