### 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)

Item *indentable = p_item;
	while (indentable) {
		if (indentable->type == ITEM_INDENT || indentable->type == ITEM_LIST) {
			return indentable;
		}
		indentable = indentable->parent;
	}
	return indentable;
}
RichTextLabel::ItemFont *RichTextLabel::_find_font(Item *p_item) {
	Item *fontitem = p_item;
	while (fontitem) {
		if (fontitem->type == ITEM_FONT) {
			ItemFont *fi = static_cast<ItemFont *>(fontitem);
			switch (fi->def_font) {
				case NORMAL_FONT: {
					if (fi->variation) {
						Ref<FontVariation> fc = fi->font;
						if (fc.is_valid()) {
							fc->set_base_font(theme_cache.normal_font);
						}
					} else {
						fi->font = theme_cache.normal_font;
					}
					if (fi->def_size) {
						fi->font_size = theme_cache.normal_font_size;
					}
				} break;
				case BOLD_FONT: {
					if (fi->variation) {