Can't close a window with a custom button

[ghost]

I'm running code which in simplified version looks like this

use std::{thread, time};
use druid::widget::{Button, Flex};
use druid::{AppLauncher, Widget, WindowDesc};

// Clicking a button should close the window

//// The close button close the window
//fn main () {
//    let window = WindowDesc::new(build_widget);
//    AppLauncher::with_window(window)
//        .use_simple_logger()
//        .launch(0_u32)
//        .expect("launch failed");
//
//    dbg!("GUI killed");
//}

// The close button makes the window hang
fn main () {
    dbg!("LAUNCH_GUI");

    let mut should_gui = true;
    loop {
        dbg!("LOOP ITERATION");
        if should_gui == true {
            dbg!("SHOW GUI");
            should_gui = false;

            let window = WindowDesc::new(build_widget);
            AppLauncher::with_window(window)
                .use_simple_logger()
                .launch(0_u32)
                .expect("launch failed");

            dbg!("GUI KILLED");
        }
        let sleep_time = time::Duration::from_millis(100);
        thread::sleep(sleep_time);
    }
}

fn build_widget() -> impl Widget<u32> {
    Flex::column()
        .with_child(
            Button::sized(
                "Close",
                |evt_ctx , data: _, _env| {
                    dbg!(&data);
                    //evt_ctx.window().close();
                    evt_ctx.submit_command(druid::commands::CLOSE_WINDOW, None);
                },
                80.0,
                20.0,
            ),
            0.0,
        )
}

I'm expecting the window is closed properly and the program keeps looping without GUI.

Instead, I'm getting a hanging window which can't be closed, see

Also discussed here: https://xi.zulipchat.com/#narrow/stream/147926-druid/topic/evt_ctx.2Ewindow().2Eclose().20not.20working.3F

Software versions

gtk3-devel.x86_64 3.24.13-1.fc31
glib2-devel.x86_64 2.62.3-1.fc31