### 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)

List<Item *>::Element *E = p_from;
	while (E != nullptr) {
		ERR_CONTINUE(E->get()->type != ITEM_FRAME); 
		ItemFrame *frame = static_cast<ItemFrame *>(E->get());
		if (p_reverse_search) {
			for (int i = (int)frame->lines.size() - 1; i >= 0; i--) {
				if (_search_line(frame, i, p_string, -1, p_reverse_search)) {
					return true;
				}
			}
		} else {
			for (int i = 0; i < (int)frame->lines.size(); i++) {
				if (_search_line(frame, i, p_string, 0, p_reverse_search)) {
					return true;
				}
			}
		}
		E = p_reverse_search ? E->prev() : E->next();
	}
	return false;
}
bool RichTextLabel::_search_line(ItemFrame *p_frame, int p_line, const String &p_string, int p_char_idx, bool p_reverse_search) {