### Godot version

v4.0.beta16.official [518b9e580]

### System information

Windows 10

### Issue description

`RichTextLabel` with `threaded` enabled may crash the game or show the following warning if the node is in another scene:

```
W 0:00:03:0802   test.gd:12 @ load_with_scene(): A Thread object has been re-started without wait_to_finish() having been called on it. Please do so to ensure correct cleanup of the thread.
  <C++ Source>   core/os/thread.cpp:76 @ start()
  <Stack Trace>  test.gd:12 @ load_with_scene()
                 test.gd:6 @ load_items()
                 test.gd:26 @ _on_button_pressed()
```

It may crash the game (without any warning) if it's added directly.

In `v4.0.beta15.official [4fa6edc88]` I was able to reproduce the same warning on my project but I can't reproduce it in the attached MRP. I get a different error and will crash too if the node is added directly:

```
E 0:00:03:0019   remove_from_group: Condition "!E" is true.
  <C++ Source>   scene/main/scene_tree.cpp:157 @ remove_from_group()
```


### Steps to reproduce

Click on the  `Load with scene` or `Load with node` buttons in the attached MRP until it shows a warning/error or crashes. It may take a while.

### Minimal reproduction project

[add_chid_bug_b16.zip](https://github.com/godotengine/godot/files/10521402/add_chid_bug_b16.zip)

Size2 sb_min_size = theme_cache.normal_style->get_minimum_size();
	Size2 min_size;
	if (fit_content) {
		min_size.x = get_content_width();
		min_size.y = get_content_height();
	}
	return sb_min_size +
			((autowrap_mode != TextServer::AUTOWRAP_OFF) ? Size2(1, min_size.height) : min_size);
}
void RichTextLabel::_generate_context_menu() {
	if (!menu) {
		menu = memnew(PopupMenu);
		add_child(menu, false, INTERNAL_MODE_FRONT);
		menu->connect("id_pressed", callable_mp(this, &RichTextLabel::_menu_option));
	}
	
	menu->clear();
	if (selection.enabled) {
		menu->add_item(RTR("Copy"), MENU_COPY, is_shortcut_keys_enabled() ? _get_menu_action_accelerator("ui_copy") : Key::NONE);
		menu->add_item(RTR("Select All"), MENU_SELECT_ALL, is_shortcut_keys_enabled() ? _get_menu_action_accelerator("ui_text_select_all") : Key::NONE);
	}
}
Key RichTextLabel::_get_menu_action_accelerator(const String &p_action) {