Using a ComboBox to determine what Form will display a database in Visual Studio

I have a Button that checks a ComboxBox that has two values, namely Lec and Lab. I want to use the ComboBox to determine what Form I want to display a database table in. I keep getting an error in SQL

An unhandled exception of type 'System.NullReferenceException' occurred in System.Data.SQLite.dll

Additional information: Object reference not set to an instance of an object.

What is wrong with reading my database?

This is the Button:

Private Sub Button5_Click(sender As Object, e As EventArgs) Handles Button5.Click
    Dim Checker As String
    Dim Checker2 As String
    Dim ClassIntl = TextBox1.Text
    SeatPlanLab.TextBox1.Text = ClassIntl
    SeatPlanCorner.TextBox1.Text = ClassIntl
    SeatPlanNotCorner.TextBox1.Text = ClassIntl
    Me.Hide()
    Checker = ComboBox1.SelectedItem.ToString
    'MessageBox.Show(Checker)
    DBConn()
    SQLSTR = "SELECT SeatPlan FROM MasterClasslist WHERE ClassID = '" & ClassIntl & "'"
    readDB()
    While (SQLDR.Read())
        Checker2 = SQLDR("SeatPlan")
        If Checker Is "Lab" Then
            SeatPlanLab.Arrangement()
            SeatPlanLab.Show()
        Else
            If Checker2 Is "corner" Then
                SeatPlanCorner.Arrangement()
                SeatPlanCorner.Show()
            Else
                SeatPlanNotCorner.Arrangement()
                SeatPlanNotCorner.Show()
            End If
        End If
    End While
    SQLDR.Dispose()
    SQLCONN.Close()
End Sub