Problem with ShowDialog() method in C#.

I want to call a form to another form, but I want the calling form shows like dialog box(not resizing). I passing parameter to my new_customer form's constructor.
so My code is :-

new_customer NC = new new_customer(null);
            NC.ShowDialog();

My new_customer form constructor :-

public new_customer(string formType="save")
       {
           InitializeComponent();
           DC = new Data_Conn(Application.StartupPath);
           DC1 = new Data_Conn(Application.StartupPath);
           paraModi = formType;
       }

Problem is when I call new_customer form from another form, its constructor doesn't execute,that's why I get an error when I use that variable :

Object reference not set to an instance of an object.

thanks In advanced ..

Tags: C#, Windows Forms