### 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)

underline_hint = p_underline;
	queue_redraw();
}
bool RichTextLabel::is_hint_underlined() const {
	return underline_hint;
}
void RichTextLabel::set_offset(int p_pixel) {
	vscroll->set_value(p_pixel);
}
void RichTextLabel::set_scroll_active(bool p_active) {
	if (scroll_active == p_active) {
		return;
	}
	scroll_active = p_active;
	vscroll->set_drag_node_enabled(p_active);
	queue_redraw();
}
bool RichTextLabel::is_scroll_active() const {
	return scroll_active;
}
void RichTextLabel::set_scroll_follow(bool p_follow) {
	scroll_follow = p_follow;
	if (!vscroll->is_visible_in_tree() || vscroll->get_value() >= (vscroll->get_max() - vscroll->get_page())) {
		scroll_following = true;
	}
}
bool RichTextLabel::is_scroll_following() const {
	return scroll_follow;
}
void RichTextLabel::parse_bbcode(const String &p_bbcode) {