### 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 outline_size;
		Color outline_color;
		Ref<Font> bold_font;
		int bold_font_size;
		Ref<Font> bold_italics_font;
		int bold_italics_font_size;
		Ref<Font> italics_font;
		int italics_font_size;
		Ref<Font> mono_font;
		int mono_font_size;
		int table_h_separation;
		int table_v_separation;
		Color table_odd_row_bg;
		Color table_even_row_bg;
		Color table_border;
		float base_scale = 1.0;
	} theme_cache;
public:
	String get_parsed_text() const;
	void add_text(const String &p_text);
	void add_image(const Ref<Texture2D> &p_image, const int p_width = 0, const int p_height = 0, const Color &p_color = Color(1.0, 1.0, 1.0), InlineAlignment p_alignment = INLINE_ALIGNMENT_CENTER, const Rect2 &p_region = Rect2(0, 0, 0, 0));
	void add_newline();
	bool remove_paragraph(const int p_paragraph);
	void push_dropcap(const String &p_string, const Ref<Font> &p_font, int p_size, const Rect2 &p_dropcap_margins = Rect2(), const Color &p_color = Color(1, 1, 1), int p_ol_size = 0, const Color &p_ol_color = Color(0, 0, 0, 0));
	void _push_def_font(DefaultFont p_def_font);
	void _push_def_font_var(DefaultFont p_def_font, const Ref<Font> &p_font, int p_size = -1);
	void push_font(const Ref<Font> &p_font, int p_size = 0);
	void push_font_size(int p_font_size);
	void push_outline_size(int p_font_size);
	void push_normal();