### 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 (p_region.has_area()) {
				item->size.height = p_region.get_size().height * p_width / p_region.get_size().width;
			} else {
				item->size.height = p_image->get_height() * p_width / p_image->get_width();
			}
		}
	} else {
		if (p_height > 0) {
			
			item->size.height = p_height;
			
			if (p_region.has_area()) {
				item->size.width = p_region.get_size().width * p_height / p_region.get_size().height;
			} else {
				item->size.width = p_image->get_width() * p_height / p_image->get_height();
			}
		} else {
			if (p_region.has_area()) {
				
				item->size = p_region.get_size();
			} else {
				
				item->size = p_image->get_size();
			}
		}
	}
	_add_item(item, false);
}
void RichTextLabel::add_newline() {