### 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)

// Get NFKD normalizer singleton.
	const UNormalizer2 *unorm = unorm2_getNFKDInstance(&err);
	ERR_FAIL_COND_V_MSG(U_FAILURE(err), TextServer::strip_diacritics(p_string), u_errorName(err));
	// Convert to UTF-16.
	Char16String utf16 = p_string.utf16();
	// Normalize.
	Vector<char16_t> normalized;
	err = U_ZERO_ERROR;
	int32_t len = unorm2_normalize(unorm, utf16.get_data(), -1
	ERR_FAIL_COND_V_MSG(err != U_BUFFER_OVERFLtServer::strip_diacritics(p_string), u_errorName(err));
	normalized.resize(len);
	err = U_ZERO_ERROR;
	unorm2_normalize(unorm, utf16.get_data(), -1, normalized.ptrw(), len, &err);
	ERR_FAIL_COND_V_MSG(U_FAILURE(err), TextServer::strip_diacritics(p_string), u_errorName(err));
	// Convert back to UTF-32.
	String normalized_string = String::utf16(normalized.ptr(), len);
	// Strip combining characters.
	String result;
	for (int i = 0; i < normalized_string.length(); i++) {
		if (u_getCombiningClass(normalized_string[i]
#ifdef GDEXTENSION
			result zed_string[i]);
#else
			result = result + normalized_string[i];
#endif
		}
	}
	return result;
}
String TextServerAdvanced::_string_to_upper(const String &p_string, const String &p_language) const {