从访问数据库vb绘制时出错

Question

我在尝试检查数据库中的值并从数据库读取文本框时遇到错误。在for i as integer行，我收到以下错误消息：

  

未处理的类型＆＃39; System.NullReferenceException＆＃39;尽管代码工作较早，但仍发生在VBPROJECT.exe中。

以下是代码：

Imports System.Data.OleDb
Public Class StudentLogin
    Dim objConnection As New OleDb.OleDbConnection(
    "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=Customers.accdb")
    Dim objDA As _
    New OleDb.OleDbDataAdapter("Select * from tblStudents", objConnection)
    Dim objCB As New OleDb.OleDbCommandBuilder(objDA)
    Dim objDS As New DataSet()
    Public UserID, Password As String
    Dim StudentFound As Boolean

    Private Sub Retrieve()
        objDS.Clear()
        objDA.FillSchema(objDS, SchemaType.Source, "tblStudents")
        objDA.Fill(objDS, "tblStudents")
    End Sub

    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles btnLogin1.Click
        UserID = txtUsername.Text
        Password = txtPassword.Text
        For i As Integer = 1 To (objDS.Tables("tblStudents").Rows.Count)
            If CStr(objDS.Tables("tblStudents").Rows(i - 1).Item("Surname")) =
           UserID And
            CStr(objDS.Tables("tblStudents").Rows(i - 1).Item("Student Number")) =
            Password Then


                StudentFound = True

                frmStudentPortal.txtMCQ.Text = CStr(objDS.Tables("tblstudents").Rows(i - 1).Item("MCQ Result"))
                frmStudentPortal.txtProject.Text = CStr(objDS.Tables("tblstudents").Rows(i - 1).Item("Project Result"))
                frmStudentPortal.txtOverall.Text = CStr(objDS.Tables("tblstudents").Rows(i - 1).Item("Overall Result"))
                frmStudentPortal.lblname.Text = "Welcome " & CStr(objDS.Tables("tblstudents").Rows(i - 1).Item("First Name"))
                frmStudentPortal.lblname.Visible = True

                MessageBox.Show("Welcome to the System!")
                lblStudentIDincorrect.Hide()
                lblPasswordIncorrect.Hide()
                Me.Hide()


                frmStudentPortal.Show()


                'Main_Page.Show()
                Exit For
            End If
            txtUsername.Text = String.Empty
            txtPassword.Text = String.Empty
        Next
        If StudentFound = False Then
            MessageBox.Show("Access Denied")
            lblStudentIDincorrect.Show()
            lblPasswordIncorrect.Show()
        End If
    End Sub