public partial class Form1 : MetroFramework.Forms.MetroForm
{
    public Form1()
    {
        InitializeComponent();
    }
    SqlConnection con = new SqlConnection(@"Data Source=(LocalDB)v11.0;AttachDbFilename=C:UsersIgurekDocumentsDatabase.mdf;Integrated Security=True;Connect Timeout=30");
    private void metroButton1_Click(object sender, EventArgs e)
    {
        this.Close();
    }

    private void metroButton2_Click(object sender, EventArgs e)
    {
        SqlDataAdapter sdf = new SqlDataAdapter("Select Count(*) From Login where Username='" + metroTextBox1.Text + "' and Password='" + metroTextBox2.Text + "'", con);
        DataTable dt = new DataTable();


        sdf.Fill(dt);  // <--------- Here's the exception
        if (dt.Rows.Count == 1)
        {
            Form1 f1 = new Form1();
            f1.Show();

        }
        else
        {
            MessageBox.Show("Sprawdz Nazwe i Haslo");
        }
    }
}