Error drawing from access database vb

Question

I keep getting an error when trying to check values from a database and read from a database to text boxes. At the for i as integer line I get the following error message:

An unhandled exception of type 'System.NullReferenceException' occurred in VBPROJECT.exe despite the code working earlier.

Here's the code:

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