Why when I close a sub-form in a Windows forms Add-in on Revit , it closes the entire add-in?

Question

I have a Windows Forms Add-in on Revit using Revit-API and C#. The project contains two forms ( Main form and About form). the About form opens by a button click in the main form. The problem is that when I try to close the About form through the Control box or by using a button inside it About.ActiveForm.Close() it automatically closes all the Forms. I tried About.ActiveForm.Hide() and also doesn't work. Also, I tried to link the Cancel Button of the about form to that button and it didn't work.

Code of initiating the About form

private void button3_Click(object sender, EventArgs e)
{
    About abt = new About();
    abt.ShowDialog(this);
}

Code of Closing the About Form

private void button3_Click(object sender, EventArgs e)
{
    this.Close();   
}

I also tried this for the closing part

private void button3_Click(object sender, EventArgs e)
{
    Close();   
}

and I tried this also for the initiating part

private void button3_Click(object sender, EventArgs e)
{
    About abt = new About();
    abt.ShowDialog();
}