### 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)

int visible_line_count = 0;
	int tab_size = 4;
	bool underline_meta = true;
	bool underline_hint = true;
	bool use_selected_font_color = false;
	HorizontalAlignment default_alignment = HORIZONTAL_ALIGNMENT_LEFT;
	ItemMeta *meta_hovering = nullptr;
	Variant current_meta;
	Array custom_effects;
	void _invalidate_current_line(ItemFrame *p_frame);
	static void _thread_function(void *self);
	void _stop_thread();
	bool _validate_line_caches();
	void _process_line_caches();
	void _add_item(Item *p_item, bool p_enter = false, bool p_ensure_newline = false);
	void _remove_item(Item *p_item, const int p_line, const int p_subitem_line);
	String language;
	TextDirection text_direction = TEXT_DIRECTION_AUTO;
	TextServer::StructuredTextParser st_parser = TextServer::STRUCTURED_TEXT_DEFAULT;
	Array st_args;
	struct Selection {
		ItemFrame *click_frame = nullptr;
		int click_line = 0;
		Item *click_item = nullptr;
		int click_char = 0;
		ItemFrame *from_frame = nullptr;
		int from_line = 0;
		Item *from_item = nullptr;
		int from_char = 0;
		ItemFrame *to_frame = nullptr;