### 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)

indent_level++;
			push_indent(indent_level);
			pos = brk_end + 1;
			tag_stack.push_front(tag);
		} else if (tag == "p") {
			push_paragraph(HORIZONTAL_ALIGNMENT_LEFT);
			pos = brk_end + 1;
			tag_stack.push_front("p");
		} else if (tag.begins_with("p ")) {
			Vector<String> subtag = tag.substr(2, tag.length()).split(" ");
			HorizontalAlignment alignment = HORIZONTAL_ALIGNMENT_LEFT;
			Control::TextDirection dir = Control::TEXT_DIRECTION_INHERITED;
			String lang;
			TextServer::StructuredTextParser st_parser_type = TextServer::STRUCTURED_TEXT_DEFAULT;
			for (int i = 0; i < subtag.size(); i++) {
				Vector<String> subtag_a = subtag[i].split("=");
				if (subtag_a.size() == 2) {
					if (subtag_a[0] == "align") {
						if (subtag_a[1] == "l" || subtag_a[1] == "left") {
							alignment = HORIZONTAL_ALIGNMENT_LEFT;
						} else if (subtag_a[1] == "c" || subtag_a[1] == "center") {
							alignment = HORIZONTAL_ALIGNMENT_CENTER;
						} else if (subtag_a[1] == "r" || subtag_a[1] == "right") {
							alignment = HORIZONTAL_ALIGNMENT_RIGHT;
						} else if (subtag_a[1] == "f" || subtag_a[1] == "fill") {
							alignment = HORIZONTAL_ALIGNMENT_FILL;
						}
					} else if (subtag_a[0] == "dir" || subtag_a[0] == "direction") {
						if (subtag_a[1] == "a" || subtag_a[1] == "auto") {
							dir = Control::TEXT_DIRECTION_AUTO;