### 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)

a.append(Color::html(values[j]));
			} else if (!nodepath.search(values[j]).is_null()) {
				if (values[j].begins_with("$")) {
					String v = values[j].substr(1, values[j].length());
					a.append(NodePath(v));
				}
			} else if (!boolean.search(values[j]).is_null()) {
				if (values[j] == "true") {
					a.append(true);
				} else if (values[j] == "false") {
					a.append(false);
				}
			} else if (!decimal.search(values[j]).is_null()) {
				a.append(values[j].to_float());
			} else if (!numerical.search(values[j]).is_null()) {
				a.append(values[j].to_int());
			} else {
				a.append(values[j]);
			}
		}
#endif
		if (values.size() > 1) {
			d[key] = a;
		} else if (values.size() == 1) {
			d[key] = a[0];
		}
	}
	return d;
}
RichTextLabel::RichTextLabel(const String &p_text) {