### 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)

int limit = 0;
			for (int i = 0; i < sd->text.length(); i++) {
				if (is_whitespace(data[i])) {
					int ks = _generate_kashida_justification_opportunies(sd->text, limit, i) + sd->start;
					if (ks != -1) {
						sd->jstops[ks] = true;
					}
					limit = i + 1;
				}
			}
			int ks = _generate_kashida_justification_opportunies(sd->text, limit, sd->text.length()) + sd->start;
			if (ks != -1) {
				sd->jstops[ks] = true;
			}
		} else {
			int limit = 0;
			while (ubrk_next(bi) != UBRK_DONE) {
				if (ubrk_getRuleStatus(bi) != UBRK_WORD_NONE) {
					int i = _convert_pos(sd, ubrk_current(bi));
					sd->jstops[i + sd->start] = false;
					int ks = _generate_kashida_justification_opportunies(sd->text, limit, i);
					if (ks != -1) {
						sd->jstops[ks + sd->start] = true;
					}
					limit = i;
				}
			}
			ubrk_close(bi);
		}