### 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)

line = p_text.substr(pos, end - pos);
		}
		if (line.length() > 0) {
			if (current->subitems.size() && current->subitems.back()->get()->type == ITEM_TEXT) {
				
				ItemText *ti = static_cast<ItemText *>(current->subitems.back()->get());
				ti->text += line;
				_invalidate_current_line(main);
			} else {
				
				ItemText *item = memnew(ItemText);
				item->text = line;
				_add_item(item, false);
			}
		}
		if (eol) {
			ItemNewline *item = memnew(ItemNewline);
			item->line = current_frame->lines.size();
			_add_item(item, false);
			current_frame->lines.resize(current_frame->lines.size() + 1);
			if (item->type != ITEM_NEWLINE) {
				current_frame->lines[current_frame->lines.size() - 1].from = item;
			}
			_invalidate_current_line(current_frame);
		}
		pos = end + 1;
	}
	queue_redraw();
}
void RichTextLabel::_add_item(Item *p_item, bool p_enter, bool p_ensure_newline) {