vb.net 链接accdb数据库出的问题

用VS2019建的项目
VB.net webforms 链接ACC数据库一直有错误，但有找不到出错在哪？哪位高手能解决一下，感激不尽！

一直提示
[NullReferenceException: 未将对象引用设置到对象的实例。]
Dim count As Integer = Commonmodule.mytable.Rows.Count
这句出错了

这个是一个Commonmodule模块

Imports System.Data.OleDb

Module Commonmodule

Public mydap As OleDbDataAdapter
Public mydataset As DataSet = New DataSet
Public mytable As DataTable
Public myrow As DataRow
Public mycmdbuilder As OleDbCommandBuilder

Public Function SQLOpera(ByVal str As String, ByVal table As String)
    Try
        Dim row As Integer = 0
        Dim selectstring As String
        selectstring = "provider=Microsoft.ACE.OLEDB.12.0;Data Source="
        selectstring = selectstring & "E:\程序\App_Data\biaojia.accd"
        mydap = New OleDbDataAdapter(str, selectstring)
        mydataset.Clear()
        mydataset.Tables.Clear()
        mydap.Fill(mydataset, table)
        mytable = mydataset.Tables.Item(0)
        myrow = mytable.Rows.Item(row)
    Catch ex As Exception
    End Try
    Return Nothing
End Function

End Module

这个是webforms 代码

Public Class WebForm1

Inherits System.Web.UI.Page
Public currentrow As Integer
Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load

End Sub

Protected Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click

     If TextBox_Logname.Text = "" Then
        MsgBox("请输入用户名！", MsgBoxStyle.OkOnly + MsgBoxStyle.Information, "提示")
        TextBox_Logname.Focus()
        Exit Sub

    ElseIf TextBox_Logpw.Text = "" Then
        MsgBox("请输入密码！", MsgBoxStyle.OkOnly + MsgBoxStyle.Information, "提示")
        TextBox_Logpw.Focus()
        Exit Sub
    End If
    Try

        Dim tablename As String = "denglu"
        Dim strr As String
        strr = "select username,pwd from denglu where (username='" & TextBox_Logname.Text & "' and pwd='" & TextBox_Logpw.Text & "')"
        Commonmodule.SQLOpera(strr, tablename)
        Dim count As Integer = Commonmodule.mytable.Rows.Count
        If count < 1 Then

            MsgBox("密码或用户名错误！请输入正确的用户名和密码！", MsgBoxStyle.OkOnly + MsgBoxStyle.Exclamation, "提示")
        Else
            Session("name") = TextBox_Logname.Text
            Response.Redirect("~/App_prg/index.aspx")

        End If

    Catch ex As Exception
        MsgBox(ex.Message)
    End Try



End Sub

End Class

Commonmodule.mytable
这个在哪里定义的
它可能是Nothing，你没有装入数据