private void mnuAddCate_Click(object sender, EventArgs e)
 {
     if (addCate != null)
     {
         addCate.Show();
     }
     else
     {
         addCate           = new AddCategory();
         addCate.MdiParent = clsFormProvider.mainF;
         addCate.Show();
         addCate.FormClosing += addCate_FormClosing;
     }
 }
 public void addCate_FormClosing(object sender, FormClosingEventArgs e)
 {
     addCate = null;
 }