### 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)

Vector2i fbg_index = Vector2i(end, start);
	Color last_color = Color(0, 0, 0, 0);
	bool draw_box = false;
	int hpad = get_theme_constant(SNAME("text_highlight_h_padding"));
	int vpad = get_theme_constant(SNAME("text_highlight_v_padding"));
	
	for (int i = start; i < end; i++) {
		Item *it = _get_item_at_pos(it_from, it_to, i);
		Color color;
		if (fbg_flag == 0) {
			color = _find_bgcolor(it);
		} else {
			color = _find_fgcolor(it);
		}
		bool change_to_color = ((color.a > 0) && ((last_color.a - 0.0) < 0.01));
		bool change_from_color = (((color.a - 0.0) < 0.01) && (last_color.a > 0.0));
		bool change_color = (((color.a > 0) == (last_color.a > 0)) && (color != last_color));
		if (change_to_color) {
			fbg_index.x = MIN(i, fbg_index.x);
			fbg_index.y = MAX(i, fbg_index.y);
		}
		if (change_from_color || change_color) {