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重新打开项目后提示未将对象引用到实例

错误所在事件：窗口,窗口1,AfterLoad

详细错误信息：

未将对象引用设置到对象的实例。

Dim dt As DataTable

Dim b As New GroupTableBuilder("统计表1",DataTables("采购"))

b.Groups.AddDef("入库日期",DateGroupEnum.Year,"年")

b.Groups.AddDef("入库日期",DateGroupEnum.Month,"月")

b.Groups.AddDef("存货名称")

b.Groups.AddDef("供应商")

b.Totals.AddDef("存货编号",AggregateEnum.Count,"订单数")

dt = b.Build(True)

Dim trv As WinForm.TreeView = e.Form.Controls("TreeView1")

trv.BuildTree(dt, "年|月|存货名称|供应商")

trv.StopRedraw

For Each nd As WinForm.TreeNode In trv.AllNodes

Dim Year As Integer = nd.DataRow("年")

Dim Month As Integer = nd.DataRow("月")

Dim Product As String = nd.DataRow("存货名称")

Dim gys As String = nd.DataRow("供应商")

Select Case nd.Level

Case 0

nd.Text = nd.text & "年(" & dt.Compute("Sum(订单数)","年 = " & Year) & "条)"

Case 1

nd.Text = nd.text & "月(" & dt.Compute("Sum(订单数)","年 = " & Year & " And 月 = " & Month) & "条)"

Case 2

nd.Text = nd.text & "(" & nd.DataRow("订单数") & "条)"

Case 3

nd.Text = nd.text & "(" & nd.DataRow("订单数") & "条)"

End Select

trv.Nodes.Insert("显示所有行","显示所有行(" & dt.Compute("Sum(订单数)") & "条)",0)

trv.ResumeRedraw

请教错误在哪，如何修改？

主题：未将对象引用设置到对象的实例。