GUI problem

[SephX]

Hello, I wonder if anyone could help me with this problem, i'm barely starting to program using GUI's and I'm kinda stuck.
I am working on this program were i create a window with two input boxes and three buttons, a triangle, rectangle and square button, and when pressed they will draw the corresponding shape in the coordinates on the input boxes. So far i have this

struct Lines_window : Window {
    Lines_window(Point xy, int w, int h, const string& title );
private:

	Closed_polyline triangle;

	In_box next_x;
	In_box next_y;
	Button quit_button;
	Menu shape_menu;

	void quit();
	void triangle_pressed();

	static void cb_tri(Address, Address);
	static void cb_quit(Address, Address);
};

Lines_window::Lines_window(Point xy, int w, int h, const string& title)
	:Window(xy,w,h,title),
	shape_menu(Point(x_max()-70,20),70,20,Menu::vertical,"shape"),
	quit_button(Point(x_max()-70,0),70,20,"Quit", cb_quit),
	next_x(Point(x_max()-310,0),50,20,"next x:"),
	next_y(Point(x_max()-210,0),50,20,"next y:")
{
	shape_menu.attach(new Button(Point(0,0),0,0,"triangle",cb_tri));
	attach(shape_menu);
	attach(quit_button);
	attach(next_y);
	attach(next_x);
	attach(triangle);
}

void Lines_window::cb_quit(Address, Address pw)
{
	reference_to<Lines_window>(pw).quit();
}

void Lines_window::quit()
{
	hide();
}

void Lines_window::cb_tri(Address, Address pw)
{
	reference_to<Lines_window>(pw).triangle_pressed();
}

void Lines_window::triangle_pressed()
{
	int x = next_x.get_int();
	int y = next_y.get_int();

	triangle.add(Point(x,y));
	triangle.add(Point(100,200));
	triangle.add(Point(300,300));

	redraw();
}


int main()
try {
    Lines_window win(Point(100,100),600,400,"lines");
    return gui_main();
}
catch(exception& e) {
    cerr << "exception: " << e.what() << '\n';
    return 1;
}
catch (...) {
    cerr << "Some exception\n";
    return 2;
}

The program compiles, but when i try to run it it shoots out a Segmentation fault error. I think it is the function that is called when the triangle button is pressed, but I can't think of another way of drawing the triangle. Any help would be appreciated!