### 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)

new_sd->parent = p_shaped;
	new_sd->start = p_start;
	new_sd->end = p_start + p_length;
	new_sd->orientation = sd->orientation;
	new_sd->direction = sd->direction;
	new_sd->custom_punct = sd->custom_punct;
	new_sd->para_direction = sd->para_direction;
	for (int i = 0; i < TextServer::SPACING_MAX; i++) {
		new_sd->extra_spacing[i] = sd->extra_spacing[i];
	}
	if (!_shape_substr(new_sd, sd, p_start, p_length)) {
		memdelete(new_sd);
		return RID();
	}
	return shaped_owner.make_rid(new_sd);
}
bool TextServerAdvanced::_shape_substr(ShapedTextDataAdvanced *p_new_sd, const ShapedTextDataAdvanced *p_sd, int64_t p_start, int64_t p_length) const {
	if (p_new_sd->valid) {
		return true;
	}
	p_new_sd->hb_buffer = hb_buffer_create();
	p_new_sd->line_breaks_valid = p_sd->line_breaks_valid;
	p_new_sd->justification_ops_valid = p_sd->justification_ops_valid;
	p_new_sd->sort_valid = false;
	p_new_sd->upos = p_sd->upos;
	p_new_sd->uthk = p_sd->uthk;
	if (p_length > 0) {
		p_new_sd->text = p_sd->text.substr(p_start - p_sd->start, p_length);
		p_new_sd->utf16 = p_new_sd->text.utf16();
		p_new_sd->script_iter = memnew(ScriptIterator(p_new_sd->text, 0, p_new_sd->text.length()));