### 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)

String bbcode_value;
		int main_value_pos = bbcode_name.find("=");
		if (main_value_pos > -1) {
			bbcode_value = bbcode_name.substr(main_value_pos + 1);
			bbcode_name = bbcode_name.substr(0, main_value_pos);
		}
		if (tag.begins_with("/") && tag_stack.size()) {
			bool tag_ok = tag_stack.size() && tag_stack.front()->get() == tag.substr(1, tag.length());
			if (tag_stack.front()->get() == "b") {
				in_bold = false;
			}
			if (tag_stack.front()->get() == "i") {
				in_italics = false;
			}
			if ((tag_stack.front()->get() == "indent") || (tag_stack.front()->get() == "ol") || (tag_stack.front()->get() == "ul")) {
				indent_level--;
			}
			if (!tag_ok) {
				txt += "[" + tag;
				add_text(txt);
				after_list_open_tag = false;
				after_list_close_tag = false;
				pos = brk_end;
				continue;
			}
			if (txt.is_empty() && after_list_open_tag) {
				txt = "\n"; 
			}
			after_list_open_tag = false;