### 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)

}
							fc->set_opentype_features(features);
						}
					}
				}
			}
			fc->set_base_font(font);
			if (def_font != CUSTOM_FONT) {
				_push_def_font_var(def_font, fc, fnt_size);
			} else {
				push_font(fc, fnt_size);
			}
			pos = brk_end + 1;
			tag_stack.push_front("font");
		} else if (tag.begins_with("outline_size=")) {
			int fnt_size = tag.substr(13, tag.length()).to_int();
			if (fnt_size > 0) {
				push_outline_size(fnt_size);
			}
			pos = brk_end + 1;
			tag_stack.push_front("outline_size");
		} else if (bbcode_name == "fade") {
			int start_index = 0;
			OptionMap::Iterator start_option = bbcode_options.find("start");
			if (start_option) {
				start_index = start_option->value.to_int();
			}
			int length = 10;
			OptionMap::Iterator length_option = bbcode_options.find("length");
			if (length_option) {