### 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)

context_menu_enabled = p_enabled;
}
bool RichTextLabel::is_context_menu_enabled() const {
	return context_menu_enabled;
}
void RichTextLabel::set_shortcut_keys_enabled(bool p_enabled) {
	shortcut_keys_enabled = p_enabled;
}
bool RichTextLabel::is_shortcut_keys_enabled() const {
	return shortcut_keys_enabled;
}
PopupMenu *RichTextLabel::get_menu() const {
	const_cast<RichTextLabel *>(this)->_generate_context_menu();
	return menu;
}
bool RichTextLabel::is_menu_visible() const {
	return menu && menu->is_visible();
}
String RichTextLabel::get_selected_text() const {
	if (!selection.active || !selection.enabled) {
		return "";
	}
	String txt;
	int to_line = main->first_invalid_line.load();
	for (int i = 0; i < to_line; i++) {
		txt += _get_line_text(main, i, selection);
	}
	return txt;
}
void RichTextLabel::deselect() {