### 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)

}
	FontAdvanced *fd = font_owner.get_or_null(f);
	ERR_FAIL_COND(!fd);
	MutexLock lock(fd->mutex);
	Vector2i fss = _get_size(fd, fs);
	hb_font_t *hb_font = _font_get_hb_handle(f, fs);
	double scale = _font_get_scale(f, fs);
	double sp_sp = p_sd->extra_spacing[SPACING_SPACE];
	double sp_gl = p_sd->extra_spacing[SPACING_GLYPH];
	bool last_run = (p_sd->end == p_end);
	double ea = _get_extra_advance(f, fs);
	bool subpos = (scale != 1.0) || (_font_get_subpixel_positioning(f) == SUBPIXEL_POSITIONING_ONE_HALF) || (_font_get_subpixel_positioning(f) == SUBPIXEL_POSITIONING_ONE_QUARTER) || (_font_get_subpixel_positioning(f) == SUBPIXEL_POSITIONING_AUTO && fs <= SUBPIXEL_POSITIONING_ONE_HALF_MAX_SIZE);
	ERR_FAIL_COND(hb_font == nullptr);
	hb_buffer_clear_contents(p_sd->hb_buffer);
	hb_buffer_set_direction(p_sd->hb_buffer, p_direction);
	int flags = (p_start == 0 ? HB_BUFFER_FLAG_BOT : 0) | (p_end == p_sd->text.length() ? HB_BUFFER_FLAG_EOT : 0);
	if (p_sd->preserve_control) {
		flags |= HB_BUFFER_FLAG_PRESERVE_DEFAULT_IGNORABLES;
	} else {
		flags |= HB_BUFFER_FLAG_DEFAULT;
	}
#if HB_VERSION_ATLEAST(5, 1, 0)
	flags |= HB_BUFFER_FLAG_PRODUCE_SAFE_TO_INSERT_TATWEEL;
#endif
	hb_buffer_set_flags(p_sd->hb_buffer, (hb_buffer_flags_t)flags);
	hb_buffer_set_script(p_sd->hb_buffer, p_script);
	if (p_sd->spans[p_span].language.is_empty()) {
		hb_language_t lang = hb_language_from_string(TranslationServer::get_singleton()->get_tool_locale().ascii().get_data(), -1);
		hb_buffer_set_language(p_sd->hb_buffer, lang);
	} else {