### 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 ((p_selection.to_item != nullptr) && (p_selection.to_item->index < l.from->index)) {
			continue;
		}
		if ((p_selection.from_item != nullptr) && (p_selection.from_item->index >= end_idx)) {
			continue;
		}
		if (it->type == ITEM_DROPCAP) {
			const ItemDropcap *dc = static_cast<ItemDropcap *>(it);
			txt += dc->text;
		} else if (it->type == ITEM_TEXT) {
			const ItemText *t = static_cast<ItemText *>(it);
			txt += t->text;
		} else if (it->type == ITEM_NEWLINE) {
			txt += "\n";
		} else if (it->type == ITEM_IMAGE) {
			txt += " ";
		}
	}
	if ((l.from != nullptr) && (p_frame == p_selection.to_frame) && (p_selection.to_item != nullptr) && (p_selection.to_item->index >= l.from->index) && (p_selection.to_item->index < end_idx)) {
		txt = txt.substr(0, p_selection.to_char);
	}
	if ((l.from != nullptr) && (p_frame == p_selection.from_frame) && (p_selection.from_item != nullptr) && (p_selection.from_item->index >= l.from->index) && (p_selection.from_item->index < end_idx)) {
		txt = txt.substr(p_selection.from_char, -1);
	}
	return txt;
}
void RichTextLabel::set_context_menu_enabled(bool p_enabled) {