以编程方式获取文本框控件会抛出System.NullReferenceException

Question

我有一个VB.Net Sub，它将列表中的值加载到表单上的文本框中（此表单是子所在的位置）。如果我直接引用文本框 ，请说txt01.Text = "Blah"，它可以正常工作;但如果我以编程方式引用相同的文本框，例如Me.Controls("txt01").Text = "Blah"，我会继续System.NullReferenceException。

以下是代码：

Private Sub lboxCodebooks_SelectedIndexChanged(sender As Object, e As 
EventArgs) Handles lboxCodebooks.SelectedIndexChanged

    'On Error GoTo ErrorHandler

    If lboxCodebooks.SelectedIndex < 0 Then GoTo CleanUpAndExit

    grpCodebook.Enabled = True

    Dim c As Codebook = CB.Item(lboxCodebooks.SelectedIndex)
    Dim ctlName As String
    Dim ctl As Control

    txtCodebookName.Text = c.Name

    For i As Integer = 1 To 93

        ctlName = "txt" & Format(i, "00")

        ctl = Me.Controls(ctlName) ' This sets ctl to Nothing - but why?

        ctl.Text = Mid(c.AlphaCodex, i, 1) ' NullReference error here.

    Next

CleanUpAndExit:
    On Error Resume Next

    c = Nothing

    Exit Sub

ErrorHandler:
    Call ErrorHandler(Err)
    Resume CleanUpAndExit

End Sub

我到底做错了什么？