### 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)

}
			Ref<FontVariation> fc;
			fc.instantiate();
			int fnt_size = -1;
			for (int i = 1; i < subtag.size(); i++) {
				Vector<String> subtag_a = subtag[i].split("=", true, 2);
				if (subtag_a.size() == 2) {
					if (subtag_a[0] == "name" || subtag_a[0] == "n") {
						String fnt = subtag_a[1];
						Ref<Font> font_data = ResourceLoader::load(fnt, "Font");
						if (font_data.is_valid()) {
							font = font_data;
							def_font = CUSTOM_FONT;
						}
					} else if (subtag_a[0] == "size" || subtag_a[0] == "s") {
						fnt_size = subtag_a[1].to_int();
					} else if (subtag_a[0] == "glyph_spacing" || subtag_a[0] == "gl") {
						int spacing = subtag_a[1].to_int();
						fc->set_spacing(TextServer::SPACING_GLYPH, spacing);
					} else if (subtag_a[0] == "space_spacing" || subtag_a[0] == "sp") {
						int spacing = subtag_a[1].to_int();
						fc->set_spacing(TextServer::SPACING_SPACE, spacing);
					} else if (subtag_a[0] == "top_spacing" || subtag_a[0] == "top") {
						int spacing = subtag_a[1].to_int();
						fc->set_spacing(TextServer::SPACING_TOP, spacing);
					} else if (subtag_a[0] == "bottom_spacing" || subtag_a[0] == "bt") {
						int spacing = subtag_a[1].to_int();
						fc->set_spacing(TextServer::SPACING_BOTTOM, spacing);
					} else if (subtag_a[0] == "embolden" || subtag_a[0] == "emb") {
						float emb = subtag_a[1].to_float();