### 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)

ERR_FAIL_COND_V(p_frame == nullptr, 0);
	ERR_FAIL_COND_V(p_line < 0 || p_line >= (int)p_frame->lines.size(), 0);
	Vector2 off;
	Line &l = p_frame->lines[p_line];
	MutexLock lock(l.text_buf->get_mutex());
	Item *it_from = l.from;
	Item *it_to = (p_line + 1 < (int)p_frame->lines.size()) ? p_frame->lines[p_line + 1].from : nullptr;
	if (it_from == nullptr) {
		return 0;
	}
	RID ci = get_canvas_item();
	bool rtl = (l.text_buf->get_direction() == TextServer::DIRECTION_RTL);
	bool lrtl = is_layout_rtl();
	bool trim_chars = (visible_characters >= 0) && (visible_chars_behavior == TextServer::VC_CHARS_AFTER_SHAPING);
	bool trim_glyphs_ltr = (visible_characters >= 0) && ((visible_chars_behavior == TextServer::VC_GLYPHS_LTR) || ((visible_chars_behavior == TextServer::VC_GLYPHS_AUTO) && !lrtl));
	bool trim_glyphs_rtl = (visible_characters >= 0) && ((visible_chars_behavior == TextServer::VC_GLYPHS_RTL) || ((visible_chars_behavior == TextServer::VC_GLYPHS_AUTO) && lrtl));
	int total_glyphs = (trim_glyphs_ltr || trim_glyphs_rtl) ? get_total_glyph_count() : 0;
	int visible_glyphs = total_glyphs * visible_ratio;
	Vector<int> list_index;
	Vector<ItemList *> list_items;
	_find_list(l.from, list_index, list_items);
	String prefix;
	for (int i = 0; i < list_index.size(); i++) {
		if (rtl) {
			prefix = prefix + ".";
		} else {
			prefix = "." + prefix;