Gtkmm拖放问题：放置阶段值校验失败

问题背景 #

我参考Gtkmm官方文档的拖放示例实现拖放功能，在DragSource的prepare信号回调中完成字符串向量的包装，在DropTarget的drop信号回调中做类型校验，但校验失败，控制台输出了一系列错误信息。

DragSource端实现代码 #

// ...
Glib::Value<std::vector<Glib::ustring>> value_obj;
value_obj.init(value_obj.value_type());
value_obj.set(list_of_strings);
provided_content = Gdk::ContentProvider::create(value_obj);

return provided_content;
//...

DropTarget端实现代码 #

auto drop_target = Gtk::DropTarget::create(G_TYPE_BOXED, Gdk::DragAction::MOVE);
drop_target->signal_drop().connect([this](const Glib::ValueBase& value, double x, double y) -> bool {
    if (!G_VALUE_HOLDS(value.gobj(), Glib::Value<std::vector<Glib::ustring>>::value_type())) {
        return false;
    }
    //...

控制台错误输出 #

(yae-app:23778): Gdk-CRITICAL **: 20:28:43.375: gdk_drop_finish: assertion 'priv->state == GDK_DROP_STATE_DROPPING' failed

(yae-app:23778): GLib-GObject-CRITICAL **: 20:28:44.199: ../../../gobject/gtype.c:4393: type id '0' is invalid

(yae-app:23778): GLib-GObject-CRITICAL **: 20:28:44.200: can't peek value table for type '<invalid>' which is not currently referenced

(yae-app:23778): GLib-GObject-CRITICAL **: 20:28:44.200: ../../../gobject/gvalue.c:104: cannot initialize GValue with type '(null)', this type has no GTypeValueTable implementation

(yae-app:23778): Gdk-CRITICAL **: 20:28:44.200: gdk_content_provider_get_value: assertion 'G_IS_VALUE (value)' failed

(yae-app:23778): GLib-GObject-CRITICAL **: 20:28:44.200: ../../../gobject/gtype.c:4393: type id '0' is invalid

(yae-app:23778): GLib-GObject-CRITICAL **: 20:28:44.200: can't peek value table for type '<invalid>' which is not currently referenced

(yae-app:23778): GLib-GObject-CRITICAL **: 20:28:44.200: ../../../gobject/gvalue.c:104: cannot initialize GValue with type '(null)', this type has no GTypeValueTable implementation

(yae-app:23778): Gdk-CRITICAL **: 20:28:44.200: gdk_content_provider_get_value: assertion 'G_IS_VALUE (value)' failed

(yae-app:23778): GLib-GIO-CRITICAL **: 20:28:44.200: g_task_return_error: assertion 'error != NULL' failed

(yae-app:23778): GLib-GIO-CRITICAL **: 20:28:44.200: GTask source_tag (source object: 0xc7628546b630, source tag: 0xe58941686df4) finalized without ever returning (using g_task_return_*()). This potentially indicates a bug in the program.

我的疑问 #

我完全照着示例的思路来写，但搞不懂为什么类型校验会失败，想确认我给DropTarget设置的类型是否正确。

内容的提问来源于stack exchange，提问作者xBACP