### 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);
	ItemStrikethrough *item = memnew(ItemStrikethrough);
	_add_item(item, true);
}
void RichTextLabel::push_paragraph(HorizontalAlignment p_alignment, Control::TextDirection p_direction, const String &p_language, TextServer::StructuredTextParser p_st_parser) {
	_stop_thread();
	MutexLock data_lock(data_mutex);
	ERR_FAIL_COND(current->type == ITEM_TABLE);
	ItemParagraph *item = memnew(ItemParagraph);
	item->alignment = p_alignment;
	item->direction = p_direction;
	item->language = p_language;
	item->st_parser = p_st_parser;
	_add_item(item, true, true);
}
void RichTextLabel::push_indent(int p_level) {
	_stop_thread();
	MutexLock data_lock(data_mutex);
	ERR_FAIL_COND(current->type == ITEM_TABLE);
	ERR_FAIL_COND(p_level < 0);
	ItemIndent *item = memnew(ItemIndent);
	item->level = p_level;
	_add_item(item, true, true);
}
void RichTextLabel::push_list(int p_level, ListType p_list, bool p_capitalize) {