### 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 to_line = 0;
		Item *to_item = nullptr;
		int to_char = 0;
		bool active = false; 
		bool enabled = false; 
		bool drag_attempt = false;
	};
	Selection selection;
	bool deselect_on_focus_loss_enabled = true;
	bool context_menu_enabled = false;
	bool shortcut_keys_enabled = true;
	
	PopupMenu *menu = nullptr;
	void _generate_context_menu();
	Key _get_menu_action_accelerator(const String &p_action);
	void _menu_option(int p_option);
	int visible_characters = -1;
	float visible_ratio = 1.0;
	TextServer::VisibleCharactersBehavior visible_chars_behavior = TextServer::VC_CHARS_BEFORE_SHAPING;
	bool _is_click_inside_selection() const;
	void _find_click(ItemFrame *p_frame, const Point2i &p_click, ItemFrame **r_click_frame = nullptr, int *r_click_line = nullptr, Item **r_click_item = nullptr, int *r_click_char = nullptr, bool *r_outside = nullptr, bool p_meta = false);
	String _get_line_text(ItemFrame *p_frame, int p_line, Selection p_sel) const;
	bool _search_line(ItemFrame *p_frame, int p_line, const String &p_string, int p_char_idx, bool p_reverse_search);
	bool _search_table(ItemTable *p_table, List<Item *>::Element *p_from, const String &p_string, bool p_reverse_search);
	float _shape_line(ItemFrame *p_frame, int p_line, const Ref<Font> &p_base_font, int p_base_font_size, int p_width, float p_h, int *r_char_offset);
	float _resize_line(ItemFrame *p_frame, int p_line, const Ref<Font> &p_base_font, int p_base_font_size, int p_width, float p_h);
	void _update_line_font(ItemFrame *p_frame, int p_line, const Ref<Font> &p_base_font, int p_base_font_size);
	int _draw_line(ItemFrame *p_frame, int p_line, const Vector2 &p_ofs, int p_width, const Color &p_base_color, int p_outline_size, const Color &p_outline_color, const Color &p_font_shadow_color, int p_shadow_outline_size, const Point2 &p_shadow_ofs, int &r_processed_glyphs);
	float _find_click_in_line(ItemFrame *p_frame, int p_line, const Vector2 &p_ofs, int p_width, const Point2i &p_click, ItemFrame **r_click_frame = nullptr, int *r_click_line = nullptr, Item **r_click_item = nullptr, int *r_click_char = nullptr, bool p_table = false, bool p_meta = false);
	String _roman(int p_num, bool p_capitalize) const;