主题：[求助]选择EXCEL,当有密码时出错

Dim dlg As New OpenFileDialog
dlg.Filter= "Excel 97-2003文件(*.xls)|*.xls|Excel 2007文件(*.xlsx)|*.xlsx"
If dlg.ShowDialog = DialogResult.Ok Then
    Dim cmb1 As WinForm.ComboBox = e.Form.Controls("ComboBox1")
    Dim txtb1 As WinForm.TextBox = e.Form.Controls("TextBox1")
    txtb1.value = dlg.FileName
    cmb1.Value = ""
    If txtb1.Value > "" Then
        Book = New XLS.Book(txtb1.value)
        cmb1.items.clear
        Dim str(Book.Sheets.count - 1) As String
        Dim Sheet As XLS.Sheet
        For i As Integer = 0 To Book.Sheets.count - 1
            str(i) = Book.Sheets(i).name ' str.add(Book.Sheets(i).name)
        Next
        cmb1.Items.AddRange(str)
    End If
    cmb1.Select()
End If

有密码时出错:

.NET Framework 版本：2.0.50727.3655
Foxtable 版本：2014.11.11.1
错误所在事件：窗口,Excel导入,Button1,Click
详细错误信息：
Object reference not set to an instance of an object.