Code don't work

Question

I made this code, but it doesn't work correctly. The window is created, but the button does not appear. I do not receive any error reports. Would anyone know why?

public class MyApp : Gtk.Application {

    public MyApp(){
        Object(
            application_id: "com.github.cristianofmc.hello",
            flags: ApplicationFlags.FLAGS_NONE
        );
    }

    protected override void activate () {
        var button_hello = new Gtk.Button.with_label ("Click me!");
        button_hello.margin = 12;
        button_hello.clicked.connect (() => {
            button_hello.label = "Hello World!";
            button_hello.sensitive = false;
        });
        var main_window = new Gtk.ApplicationWindow (this);
        main_window.default_height = 300;
        main_window.default_width = 300;
        main_window.title = "Hello World";
        main_window.show_all();
        main_window.add (button_hello);
    }

    public static int main (string[] args) {
        var app = new MyApp ();
        return app.run (args);
    }
}

Answer 1

You are showing all the window objects and then adding the button afterwards. Try:

main_window.add (button_hello);
main_window.show_all();