### 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(p_font_rid);
	ERR_FAIL_COND_V_MSG((p_char >= 0xd800 && p_char <= 0xdfff) || (p_char > 0x10ffff), false, "Unicode parsing error: Invalid unicode codepoint " + String::num_int64(p_char, 16) + ".");
	if (!fd) {
		return false;
	}
	MutexLock lock(fd->mutex);
	if (fd->cache.is_empty()) {
		ERR_FAIL_COND_V(!_ensure_cache_for_size(fd, fd->msdf ? Vector2i(fd->msdf_source_size, 0) : Vector2i(16, 0)), false);
	}
	FontForSizeAdvanced *at_size = fd->cache.begin()->value;
#ifdef MODULE_FREETYPE_ENABLED
	if (at_size && at_size->face) {
		return FT_Get_Char_Index(at_size->face, p_char) != 0;
	}
#endif
	return (at_size) ? at_size->glyph_map.has((int32_t)p_char) : false;
}
String TextServerAdvanced::_font_get_supported_chars(const RID &p_font_rid) const {