My new form is being closed automatically

Question

Hi,
This is from my mainForm:

        private void buttonAdd_Click(object sender, EventArgs e)

        {

            //New ContactForm

            ContactForm contactForm = new ContactForm();

        }

The ContactForm opens and then I fill in some data and I press the button called Add:

        private void buttonAddContact_Click(object sender, EventArgs e)

        {

            bool isDataOk = false;

            isDataOk = contact.AddAddress(textBoxFirstName.Text,

                                            textBoxLastName.Text,

                                            textBoxPhoneHome.Text,

                                            textBoxPhoneCell.Text,

                                            textBoxEmailPrivate.Text,

                                            textBoxEmailBusiness.Text,

                                            textBoxStreet.Text,

                                            textBoxCity.Text,

                                            textBoxZipcode.Text,

                                            comboBoxCountries.Text);

        }

And now my Form closes which seems to be logical. However I want that when isDataOk = false (i.e. the input data doesn't look 100%) it should now close.

I'm not sure where and how I should put this. Can anyone please assist me in this?

Best regards,
Tomas