### 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)

RID fdef = p_fonts[0];
		if (_font_is_allow_system_fallback(fdef)) {
			String text = p_sd->text.substr(p_start, 1);
			String font_name = _font_get_name(fdef);
			BitField<FontStyle> font_style = _font_get_style(fdef);
			int font_weight = _font_get_weight(fdef);
			int font_stretch = _font_get_stretch(fdef);
			Dictionary dvar = _font_get_variation_coordinates(fdef);
			static int64_t wgth_tag = _name_to_tag("weight");
			static int64_t wdth_tag = _name_to_tag("width");
			static int64_t ital_tag = _name_to_tag("italic");
			if (dvar.has(wgth_tag)) {
				font_weight = dvar[wgth_tag].operator int();
			}
			if (dvar.has(wdth_tag)) {
				font_stretch = dvar[wdth_tag].operator int();
			}
			if (dvar.has(ital_tag) && dvar[ital_tag].operator int() == 1) {
				font_style.set_flag(TextServer::FONT_ITALIC);
			}
			char scr_buffer[5] = { 0, 0, 0, 0, 0 };
			hb_tag_to_string(hb_script_to_iso15924_tag(p_script), scr_buffer);
			String script_code = String(scr_buffer);
			String locale = (p_sd->spans[p_span].language.is_empty()) ? TranslationServer::get_singleton()->get_tool_locale() : p_sd->spans[p_span].language;
			PackedStringArray fallback_font_name = OS::get_singleton()->get_system_font_path_for_text(font_name, text, locale, script_code, font_weight, font_stretch, font_style & TextServer::FONT_ITALIC);
#ifdef GDEXTENSION
			for (int fb = 0; fb < fallback_font_name.size(); fb++) {
				const String &E = fallback_font_name[fb];
#else