/// <summary>
 /// When close user management form, close() will release current form and need to assign null to the form.
 /// If not, will throw exception.
 /// </summary>
 /// <param name="sender"></param>
 /// <param name="e"></param>
 private void FormManagerInfo_FormClosing(object sender, FormClosingEventArgs e)
 {
     _form = null;
 }