### 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)

_invalidate_current_line(current_frame);
	if (fit_content) {
		update_minimum_size();
	}
	queue_redraw();
}
void RichTextLabel::_remove_item(Item *p_item, const int p_line, const int p_subitem_line) {
	int size = p_item->subitems.size();
	if (size == 0) {
		p_item->parent->subitems.erase(p_item);
		
		if (p_item->type == ITEM_NEWLINE) {
			current_frame->lines.remove_at(p_line);
			for (int i = 0; i < current->subitems.size(); i++) {
				if (current->subitems[i]->line > p_subitem_line) {
					current->subitems[i]->line--;
				}
			}
		}
	} else {
		
		for (int i = 0; i < size; i++) {
			_remove_item(p_item->subitems.front()->get(), p_line, p_subitem_line);
		}
		
		p_item->parent->subitems.erase(p_item);
	}
	memdelete(p_item);
}
void RichTextLabel::add_image(const Ref<Texture2D> &p_image, const int p_width, const int p_height, const Color &p_color, InlineAlignment p_alignment, const Rect2 &p_region) {