主题：代码错误

我用下面的代码实现新增功能：

Dim tr As WinForm.TreeView = e.Form.Controls("TreeView1")
Dim nd As WinForm.TreeNode = tr.SelectedNode
If nd IsNot Nothing Then
    Dim pth() As String = nd.FullPath.Split("\")
    Select Case nd.Level
        Case 2
            Tables("工程主表.支付明细表").AddNew
            Tables("工程主表.支付明细表").Current("工程类别") = pth(1)
            Tables("工程主表.支付明细表").Current("工程编号") = pth(2)
    End Select
End If

运行后出现如下提示：

Object reference not set to an instance of an object.

请问需要怎么修改？请指教，谢谢！