### 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)

String txt;
	ERR_FAIL_COND_V(p_frame == nullptr, txt);
	ERR_FAIL_COND_V(p_line < 0 || p_line >= (int)p_frame->lines.size(), txt);
	Line &l = p_frame->lines[p_line];
	Item *it_to = (p_line + 1 < (int)p_frame->lines.size()) ? p_frame->lines[p_line + 1].from : nullptr;
	int end_idx = 0;
	if (it_to != nullptr) {
		end_idx = it_to->index;
	} else {
		for (Item *it = l.from; it; it = _get_next_item(it)) {
			end_idx = it->index + 1;
		}
	}
	for (Item *it = l.from; it && it != it_to; it = _get_next_item(it)) {
		if (it->type == ITEM_TABLE) {
			ItemTable *table = static_cast<ItemTable *>(it);
			for (Item *E : table->subitems) {
				ERR_CONTINUE(E->type != ITEM_FRAME); 
				ItemFrame *frame = static_cast<ItemFrame *>(E);
				for (int i = 0; i < (int)frame->lines.size(); i++) {
					txt += _get_line_text(frame, i, p_selection);