### 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)

p_item->parent = current;
	p_item->E = current->subitems.push_back(p_item);
	p_item->index = current_idx++;
	p_item->char_ofs = current_char_ofs;
	if (p_item->type == ITEM_TEXT) {
		ItemText *t = static_cast<ItemText *>(p_item);
		current_char_ofs += t->text.length();
	} else if (p_item->type == ITEM_IMAGE) {
		current_char_ofs++;
	}
	if (p_enter) {
		current = p_item;
	}
	if (p_ensure_newline) {
		Item *from = current_frame->lines[current_frame->lines.size() - 1].from;
		
		if (_find_layout_subitem(from, p_item)) {
			_invalidate_current_line(current_frame);
			current_frame->lines.resize(current_frame->lines.size() + 1);
		}
	}
	if (current_frame->lines[current_frame->lines.size() - 1].from == nullptr) {
		current_frame->lines[current_frame->lines.size() - 1].from = p_item;
	}
	p_item->line = current_frame->lines.size() - 1;