### 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)

if (deselect_on_focus_loss_enabled == p_enabled) {
		return;
	}
	deselect_on_focus_loss_enabled = p_enabled;
	if (p_enabled && selection.active && !has_focus()) {
		deselect();
	}
}
Variant RichTextLabel::get_drag_data(const Point2 &p_point) {
	if (selection.drag_attempt && selection.enabled) {
		String t = get_selected_text();
		Label *l = memnew(Label);
		l->set_text(t);
		set_drag_preview(l);
		return t;
	}
	return Variant();
}
bool RichTextLabel::_is_click_inside_selection() const {
	if (selection.active && selection.enabled && selection.click_frame && selection.from_frame && selection.to_frame) {
		const Line &l_click = selection.click_frame->lines[selection.click_line];
		const Line &l_from = selection.from_frame->lines[selection.from_line];
		const Line &l_to = selection.to_frame->lines[selection.to_line];
		return (l_click.char_offset + selection.click_char >= l_from.char_offset + selection.from_char) && (l_click.char_offset + selection.click_char <= l_to.char_offset + selection.to_char);
	} else {
		return false;
	}
}
bool RichTextLabel::_search_table(ItemTable *p_table, List<Item *>::Element *p_from, const String &p_string, bool p_reverse_search) {