### 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)

_stop_thread();
	MutexLock data_lock(data_mutex);
	ERR_FAIL_COND(current->type == ITEM_TABLE);
	ERR_FAIL_COND(p_string.is_empty());
	ERR_FAIL_COND(p_font.is_null());
	ERR_FAIL_COND(p_size <= 0);
	ItemDropcap *item = memnew(ItemDropcap);
	item->text = p_string;
	item->font = p_font;
	item->font_size = p_size;
	item->color = p_color;
	item->ol_size = p_ol_size;
	item->ol_color = p_ol_color;
	item->dropcap_margins = p_dropcap_margins;
	_add_item(item, false);
}
void RichTextLabel::_push_def_font_var(DefaultFont p_def_font, const Ref<Font> &p_font, int p_size) {
	_stop_thread();
	MutexLock data_lock(data_mutex);
	ERR_FAIL_COND(current->type == ITEM_TABLE);
	ItemFont *item = memnew(ItemFont);
	item->def_font = p_def_font;
	item->variation = true;
	item->font = p_font;
	item->font_size = p_size;
	item->def_size = (p_size <= 0);
	_add_item(item, true);
}
void RichTextLabel::_push_def_font(DefaultFont p_def_font) {