### 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)

if (U_SUCCESS(err)) {
		while (ubrk_next(bi) != UBRK_DONE) {
			int pos = _convert_pos(p_string, utf16, ubrk_current(bi)) - 1;
			if (pos != p_string.length() - 1) {
				breaks.insert(pos);
			}
		}
		ubrk_close(bi);
	}
	PackedInt32Array ret;
	int line_start = 0;
	int line_end = 0; // End of last word on current line.
	int word_start = 0; // -1 if no word encountered. Leading spaces are part of a word.
	int word_length = 0;
	for (int i = 0; i < p_string.length(); i++) {
		const char32_t c = p_string[i];
		if (is_linebreak(c)) {
			// Force newline.
			ret.push_back(line_start);
			ret.push_back(i);
			line_start = i + 1;
			line_end = line_start;
			word_start = line_start;
			word_length = 0;
		} else if (c == 0xfffc) {
			continue;
		} else if ((u_ispunct(c) && c != 0x005F) || is_underscore(c) || c == '\t' || is_whitespace(c)) {
			// A whitespace ends current word.
			if (word_length > 0) {
				line_end = i - 1;