Segfault if trying to use certain builders before GTK is initialized

[anna-hope]

(This is partially duplicated from gtk-rs/gir#1590, because I think that's where the fix should ultimately go, even though the bug itself affects gtk4-rs)

Using some builders before initializing GTK can cause a segfault. Instead, the generated builder should check if GTK has been initialized, and panic if not.

use gtk4::prelude::*;
use gtk4::{Application, ApplicationWindow, glib};

const APP_ID: &str = "me.annahope.gtk4-rs-segfault";

fn build_ui(app: &Application) {
    let window = ApplicationWindow::builder()
        .application(app)
        .title("Segfault")
        .build();  // Segfaults if GTK hasn't been initialized.

    window.present();
}

fn main() -> glib::ExitCode {
    let app = Application::builder().application_id(APP_ID).build();
    // app.connect_activate(build_ui); // Doesn't segfault, as expected
    build_ui(&app); // Segfaults.
    app.run()
}

Stacktrace

gtk_style_provider_get_settings 0x0000000101dfd1c4
gtk_css_value_initial_compute 0x0000000101e926bc
gtk_css_static_style_compute_value 0x0000000101ea2384
gtk_css_static_style_new_compute 0x0000000101ea0ebc
gtk_css_static_style_get_default 0x0000000101ea0c78
gtk_css_node_init 0x0000000101e94ca4
g_type_create_instance 0x00000001015bf428
g_object_new_internal 0x00000001015ab838
g_object_new_with_properties 0x00000001015ab0e4
gtk_css_widget_node_new 0x0000000101eaa4ac
gtk_widget_init 0x0000000101e4124c
g_type_create_instance 0x00000001015bf428
g_object_new_internal 0x00000001015ab838
g_object_new_with_properties 0x00000001015ab0b0
glib::object::Object::new_internal object.rs:1490
glib::object::Object::with_mut_values object.rs:1425
glib::object::ObjectBuilder<O>::build object.rs:1634
gtk4::auto::application_window::ApplicationWindowBuilder::build application_window.rs:420
gtk4_segfault::build_ui main.rs:7
gtk4_segfault::main main.rs:18
core::ops::function::FnOnce::call_once function.rs:250
std::sys_common::backtrace::__rust_begin_short_backtrace backtrace.rs:155
std::rt::lang_start::{{closure}} rt.rs:159
[Inlined] core::ops::function::impls::<impl core::ops::function::FnOnce<A> for &F>::call_once function.rs:284
[Inlined] std::panicking::try::do_call panicking.rs:559
[Inlined] std::panicking::try panicking.rs:523
[Inlined] std::panic::catch_unwind panic.rs:149
[Inlined] std::rt::lang_start_internal::{{closure}} rt.rs:141
[Inlined] std::panicking::try::do_call panicking.rs:559
[Inlined] std::panicking::try panicking.rs:523
[Inlined] std::panic::catch_unwind panic.rs:149
std::rt::lang_start_internal rt.rs:141
std::rt::lang_start rt.rs:158
main 0x0000000100edbbd0
start 0x000000019a4b60e0

[bilelmoussaoui]

This is now fixed