private void Button2_Click(object sender, EventArgs e)
        {
            this.Hide();
            st_registration_form stregform = new st_registration_form();

            stregform.FormClosed += (s, args) => this.Close();
            stregform.Show();
        }
        private void PictureBox8_Click(object sender, EventArgs e)
        {
            st_registration_form stregform = new st_registration_form();

            stregform.Show();
        }