Not knowing much about what's going on in the glue between native object and js proxies :(
Here is the program to reproduce :

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const Lang = imports.lang;
const Mainloop = imports.mainloop;
const Clutter = imports.gi.Clutter;

Clutter.init(null, null);

let stage = new Clutter.Stage({
    width: 200,
    height: 200,
});
stage.connect('destroy', Clutter.main_quit);

stage.add_child(new Clutter.Actor({
    width: 200,
    height: 200,
    background_color: new Clutter.Color({
        red: 0xff, green: 0, blue: 0, alpha: 0xff,
    }),
}));

Mainloop.timeout_add(2000, Lang.bind(this, function() {
    let children = stage.get_children();
    for (let i in children)	{
        let actor = children[i];
        actor.save_easing_state();
        actor.set_easing_duration(300)
        actor.set_easing_mode(Clutter.AnimationMode.LINEAR);
        actor.scale_x = actor.scale_y = 0;
        actor.restore_easing_state();

        let timeline = actor.get_transition('scale-x');
        timeline.connect('completed', Lang.bind(this, function() {
            stage.remove_child(actor);
        }));
        break;
    }
}));

stage.show();
Clutter.main();

----------------------------------------------------------------

And here is the backtrace : 

Program received signal SIGSEGV, Segmentation fault.
0x00007ffff7ba35f8 in g_object_steal_qdata (object=object@entry=0x19e1180, quark=353) at gobject.c:3443
3443	  g_return_val_if_fail (G_IS_OBJECT (object), NULL);
(gdb) bt

+ Trace 232366