### 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 processed_glyphs = 0;
			while (ofs.y < size.height && from_line < to_line) {
				MutexLock lock(main->lines[from_line].text_buf->get_mutex());
				visible_paragraph_count++;
				visible_line_count += _draw_line(main, from_line, ofs, text_rect.size.x, theme_cache.default_color, theme_cache.outline_size, theme_cache.font_outline_color, theme_cache.font_shadow_color, theme_cache.shadow_outline_size, shadow_ofs, processed_glyphs);
				ofs.y += main->lines[from_line].text_buf->get_size().y + main->lines[from_line].text_buf->get_line_count() * theme_cache.line_separation;
				from_line++;
			}
		} break;
		case NOTIFICATION_INTERNAL_PROCESS: {
			if (is_visible_in_tree()) {
				if (!is_ready()) {
					return;
				}
				double dt = get_process_delta_time();
				_update_fx(main, dt);
				queue_redraw();
			}
		} break;
		case NOTIFICATION_FOCUS_EXIT: {
			if (deselect_on_focus_loss_enabled) {
				deselect();
			}
		} break;
		case NOTIFICATION_DRAG_END: {
			selection.drag_attempt = false;
		} break;
	}
}
Control::CursorShape RichTextLabel::get_cursor_shape(const Point2 &p_pos) const {