Items collection cannot be modified when the datasource property is set.

I want to create a login form, the username and password are stored in Database access, I tried it but it failed

Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load

        ComboBox1.Items.Clear() <--This line is always pointed
        PasswordTextBox.Clear()

        Dim ctr, i As Integer
        ds.Clear()
        If cn.State = ConnectionState.Open Then
            cn.Close()
        End If
        Module1.conn()
        cn.Open()
        str = "select * from Users"
        cmd = New OleDbCommand(str, cn)
        da.SelectCommand = cmd
        da.Fill(ds, "Users")
        ctr = ds.Tables("Users").Rows.Count - 1
        For i = 0 To ctr
            ComboBox1.Items.Add(ds.Tables("Users").Rows(i)(1).ToString)
        Next

    End Sub