### 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 Rect2(theme_cache.normal_style->get_offset(), get_size() - theme_cache.normal_style->get_minimum_size());
}
RichTextLabel::Item *RichTextLabel::_get_item_at_pos(RichTextLabel::Item *p_item_from, RichTextLabel::Item *p_item_to, int p_position) {
	int offset = 0;
	for (Item *it = p_item_from; it && it != p_item_to; it = _get_next_item(it)) {
		switch (it->type) {
			case ITEM_TEXT: {
				ItemText *t = static_cast<ItemText *>(it);
				offset += t->text.length();
				if (offset > p_position) {
					return it;
				}
			} break;
			case ITEM_NEWLINE: {
				offset += 1;
				if (offset == p_position) {
					return it;
				}
			} break;
			case ITEM_IMAGE: {
				offset += 1;
				if (offset > p_position) {
					return it;
				}
			} break;