private void btnRegister_Click(object sender, EventArgs e)
 {
     if (rf == null)
     {
         rf             = new RegisterForm();
         rf.FormClosed += rf_FormClosed;
     }
     rf.Show(this);
     Hide();
 }
 private void rf_FormClosed(object sender, FormClosedEventArgs e)
 {
     rf = null;
     Show();
 }