void PopForm_FormClosed(object sender, FormClosedEventArgs e)
 {
     loginForm = null;
     clientForm = null;
     popForm = null;
 }
 void ShowPopForm(object sender, EventArgs e)
 {
     if (popForm != null)
         popForm.Activate();
     else
     {
         if (this.controller.IsLogin())
         {
             popForm = clientForm = new ClientForm(this.controller);
         }
         else
         {
             popForm = loginForm = new LoginForm(this.controller);
         }
         popForm.Show();
         popForm.Activate();
         popForm.FormClosed += PopForm_FormClosed;
     }
 }