### 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)

return visible_characters;
}
int RichTextLabel::get_character_line(int p_char) {
	_validate_line_caches();
	int line_count = 0;
	int to_line = main->first_invalid_line.load();
	for (int i = 0; i < to_line; i++) {
		MutexLock lock(main->lines[i].text_buf->get_mutex());
		if (main->lines[i].char_offset < p_char && p_char <= main->lines[i].char_offset + main->lines[i].char_count) {
			for (int j = 0; j < main->lines[i].text_buf->get_line_count(); j++) {
				Vector2i range = main->lines[i].text_buf->get_line_range(j);
				if (main->lines[i].char_offset + range.x < p_char && p_char <= main->lines[i].char_offset + range.y) {
					return line_count;
				}
				line_count++;
			}
		} else {
			line_count += main->lines[i].text_buf->get_line_count();
		}
	}
	return -1;
}
int RichTextLabel::get_character_paragraph(int p_char) {