### 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)

ERR_FAIL_COND_V_MSG(U_FAILURE(status), -1, u_errorName(status));
	return match_index;
}
bool TextServerAdvanced::_spoof_check(const String &p_string) const {
#ifndef ICU_STATIC_DATA
	if (!icu_data_loaded) {
		return false;
	}
#endif
	UErrorCode status = U_ZERO_ERROR;
	Char16String utf16 = p_string.utf16();
	USet *allowed = uset_openEmpty();
	uset_addAll(allowed, uspoof_getRecommendedSet(&status));
	uset_addAll(allowed, uspoof_getInclusionSet(&status));
	USpoofChecker *sc = uspoof_open(&status);
	uspoof_setAllowedChars(sc, allowed, &status);
	uspoof_setRestrictionLevel(sc, USPOOF_MODERATELY_RESTRICTIVE);
	int32_t bitmask = uspoof_check(sc, utf16.get_data(), -1, NULL, &status);
	uspoof_close(sc);
	uset_close(allowed);
	ERR_FAIL_COND_V_MSG(U_FAILURE(status), false, u_errorName(status));
	return (bitmask != 0);
}
String TextServerAdvanced::_strip_diacritics(const String &p_string) const {
#ifndef ICU_STATIC_DATA
	if (!icu_data_loaded) {
		return TextServer::strip_diacritics(p_string);
	}
#endif
	UErrorCode err = U_ZERO_ERROR;