both forms close unexpectedly when second form appears

I want to close first form when second is show. But all forms closes unexpectedly! I couldn't understand why. Please, explain me it's cause and help me to do this correctly.

My codes:

private void b_digerForm_Click(object sender, EventArgs e)
{
    this.Close();
    F_Diger digerForm = new F_Diger();
    digerForm.ShowDialog();
}

P.S: I'm new to C#. I searched out about this problem, but couldn't find any helpful post.

Thanks a lot.