### 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)

return false;
	}
	
	Vector<int> subitem_indices_to_remove;
	for (int i = 0; i < current->subitems.size(); i++) {
		if (current->subitems[i]->line == p_paragraph) {
			subitem_indices_to_remove.push_back(i);
		}
	}
	bool had_newline = false;
	
	for (int i = subitem_indices_to_remove.size() - 1; i >= 0; i--) {
		int subitem_idx = subitem_indices_to_remove[i];
		had_newline = had_newline || current->subitems[subitem_idx]->type == ITEM_NEWLINE;
		_remove_item(current->subitems[subitem_idx], current->subitems[subitem_idx]->line, p_paragraph);
	}
	if (!had_newline) {
		current_frame->lines.remove_at(p_paragraph);
		if (current_frame->lines.size() == 0) {
			current_frame->lines.resize(1);
		}
	}
	if (p_paragraph == 0 && current->subitems.size() > 0) {
		main->lines[0].from = main;
	}
	main->first_invalid_line.store(0);
	queue_redraw();
	return true;
}
void RichTextLabel::push_dropcap(const String &p_string, const Ref<Font> &p_font, int p_size, const Rect2 &p_dropcap_margins, const Color &p_color, int p_ol_size, const Color &p_ol_color) {