### 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)

l.offset.x = _find_margin(l.from, p_base_font, p_base_font_size);
	l.text_buf->set_width(p_width - l.offset.x);
	l.text_buf->set_alignment(_find_alignment(l.from));
	l.text_buf->set_direction(_find_direction(l.from));
	if (tab_size > 0) { 
		Vector<float> tabs;
		tabs.push_back(tab_size * p_base_font->get_char_size(' ', p_base_font_size).width);
		l.text_buf->tab_align(tabs);
	}
	
	String txt;
	Item *it_to = (p_line + 1 < (int)p_frame->lines.size()) ? p_frame->lines[p_line + 1].from : nullptr;
	int remaining_characters = visible_characters - l.char_offset;
	for (Item *it = l.from; it && it != it_to; it = _get_next_item(it)) {
		if (visible_chars_behavior == TextServer::VC_CHARS_BEFORE_SHAPING && visible_characters >= 0 && remaining_characters <= 0) {
			break;
		}
		switch (it->type) {
			case ITEM_DROPCAP: {
				
				const ItemDropcap *dc = static_cast<ItemDropcap *>(it);
				l.text_buf->set_dropcap(dc->text, dc->font, dc->font_size, dc->dropcap_margins);
				l.dc_color = dc->color;
				l.dc_ol_size = dc->ol_size;
				l.dc_ol_color = dc->ol_color;
			} break;
			case ITEM_NEWLINE: {
				Ref<Font> font = p_base_font;
				int font_size = p_base_font_size;
				ItemFont *font_it = _find_font(it);