### 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)

fbg_index.x = MIN(i, fbg_index.x);
			fbg_index.y = MAX(i, fbg_index.y);
			draw_box = true;
		}
		if (draw_box) {
			Vector<Vector2> sel = TS->shaped_text_get_selection(p_rid, fbg_index.x, fbg_index.y);
			for (int j = 0; j < sel.size(); j++) {
				Vector2 rect_off = line_off + Vector2(sel[j].x - hpad, -TS->shaped_text_get_ascent(p_rid) - vpad);
				Vector2 rect_size = Vector2(sel[j].y - sel[j].x + 2 * hpad, TS->shaped_text_get_size(p_rid).y + 2 * vpad);
				RenderingServer::get_singleton()->canvas_item_add_rect(p_ci, Rect2(rect_off, rect_size), last_color);
			}
			fbg_index = Vector2i(end, start);
			draw_box = false;
		}
		if (change_color) {
			fbg_index.x = MIN(i, fbg_index.x);
			fbg_index.y = MAX(i, fbg_index.y);
		}
		last_color = color;
	}
	if (last_color.a > 0) {
		Vector<Vector2> sel = TS->shaped_text_get_selection(p_rid, fbg_index.x, end);
		for (int i = 0; i < sel.size(); i++) {
			Vector2 rect_off = line_off + Vector2(sel[i].x - hpad, -TS->shaped_text_get_ascent(p_rid) - vpad);
			Vector2 rect_size = Vector2(sel[i].y - sel[i].x + 2 * hpad, TS->shaped_text_get_size(p_rid).y + 2 * vpad);
			RenderingServer::get_singleton()->canvas_item_add_rect(p_ci, Rect2(rect_off, rect_size), last_color);
		}
	}
}
Ref<RichTextEffect> RichTextLabel::_get_custom_effect_by_code(String p_bbcode_identifier) {