### 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)

UErrorCode err = U_ZERO_ERROR;
		int i = 0;
		while (i < sd->spans.size()) {
			String language = sd->spans[i].language;
			int r_start = sd->spans[i].start;
			while (i + 1 < sd->spans.size() && language == sd->spans[i + 1].language) {
				i++;
			}
			int r_end = sd->spans[i].end;
			UBreakIterator *bi = ubrk_open(UBRK_LINE, (language.is_empty()) ? TranslationServer::get_singleton()->get_tool_locale().ascii().get_data() : language.ascii().get_data(), data + _convert_pos_inv(sd, r_start), _convert_pos_inv(sd, r_end - r_start), &err);
			if (U_FAILURE(err)) {
				
				for (int j = r_start; j < r_end; j++) {
					char32_t c = sd->text[j - sd->start];
					if (is_whitespace(c)) {
						sd->breaks[j + 1] = false;
					}
					if (is_linebreak(c)) {
						sd->breaks[j + 1] = true;
					}
				}
			} else {
				while (ubrk_next(bi) != UBRK_DONE) {
					int pos = _convert_pos(sd, ubrk_current(bi)) + r_start;
					if ((ubrk_getRuleStatus(bi) >= UBRK_LINE_HARD) && (ubrk_getRuleStatus(bi) < UBRK_LINE_HARD_LIMIT)) {
						sd->breaks[pos] = true;
					} else if ((ubrk_getRuleStatus(bi) >= UBRK_LINE_SOFT) && (ubrk_getRuleStatus(bi) < UBRK_LINE_SOFT_LIMIT)) {
						sd->breaks[pos] = false;
					}
					int pos_p = pos - 1 - sd->start;