private void User_FormClosed(object sender, FormClosedEventArgs e)
 {
     _user = null;
 }
 private void createUserToolStripMenuItem_Click(object sender, EventArgs e)
 {
     if (_user == null)
     {
         _user = new FrmCreateUser {MdiParent = this};
         _user.FormClosed += User_FormClosed;
         _user.Show();
     }
     else
     {
         _user.Activate();
         _user.WindowState = FormWindowState.Normal;
     }
 }