private void button6_Click(object sender, EventArgs e)
{
if (string.IsNullOrWhiteSpace(textBox6.Text))
{
PopUp.ShowInfo("textbox is empty");
return;
}
if (listBox3.Items.Contains(textBox6.Text))
{
PopUp.ShowInfo($"{textBox6.Text} is already added");
return;
}
listBox3.Items.Add(textBox6.Text);
textBox6.Clear();
}