### 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)

case ITEM_TABLE: {
				offset += 1;
			} break;
			default:
				break;
		}
	}
	return p_item_from;
}
String RichTextLabel::_roman(int p_num, bool p_capitalize) const {
	if (p_num > 3999) {
		return "ERR";
	};
	String s;
	if (p_capitalize) {
		const String roman_M[] = { "", "M", "MM", "MMM" };
		const String roman_C[] = { "", "C", "CC", "CCC", "CD", "D", "DC", "DCC", "DCCC", "CM" };
		const String roman_X[] = { "", "X", "XX", "XXX", "XL", "L", "LX", "LXX", "LXXX", "XC" };
		const String roman_I[] = { "", "I", "II", "III", "IV", "V", "VI", "VII", "VIII", "IX" };
		s = roman_M[p_num / 1000] + roman_C[(p_num % 1000) / 100] + roman_X[(p_num % 100) / 10] + roman_I[p_num % 10];
	} else {
		const String roman_M[] = { "", "m", "mm", "mmm" };
		const String roman_C[] = { "", "c", "cc", "ccc", "cd", "d", "dc", "dcc", "dccc", "cm" };
		const String roman_X[] = { "", "x", "xx", "xxx", "xl", "l", "lx", "lxx", "lxxx", "xc" };
		const String roman_I[] = { "", "i", "ii", "iii", "iv", "v", "vi", "vii", "viii", "ix" };
		s = roman_M[p_num / 1000] + roman_C[(p_num % 1000) / 100] + roman_X[(p_num % 100) / 10] + roman_I[p_num % 10];
	}
	return s;
}
String RichTextLabel::_letters(int p_num, bool p_capitalize) const {