### 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)

ERR_FAIL_COND_V(p_frame == nullptr, p_h);
	ERR_FAIL_COND_V(p_line < 0 || p_line >= (int)p_frame->lines.size(), p_h);
	Line &l = p_frame->lines[p_line];
	MutexLock lock(l.text_buf->get_mutex());
	BitField<TextServer::LineBreakFlag> autowrap_flags = TextServer::BREAK_MANDATORY;
	switch (autowrap_mode) {
		case TextServer::AUTOWRAP_WORD_SMART:
			autowrap_flags = TextServer::BREAK_WORD_BOUND | TextServer::BREAK_ADAPTIVE | TextServer::BREAK_MANDATORY;
			break;
		case TextServer::AUTOWRAP_WORD:
			autowrap_flags = TextServer::BREAK_WORD_BOUND | TextServer::BREAK_MANDATORY;
			break;
		case TextServer::AUTOWRAP_ARBITRARY:
			autowrap_flags = TextServer::BREAK_GRAPHEME_BOUND | TextServer::BREAK_MANDATORY;
			break;
		case TextServer::AUTOWRAP_OFF:
			break;
	}
	autowrap_flags = autowrap_flags | TextServer::BREAK_TRIM_EDGE_SPACES;
	
	l.text_buf->clear();
	l.text_buf->set_break_flags(autowrap_flags);
	l.text_buf->set_justification_flags(TextServer::JUSTIFICATION_KASHIDA | TextServer::JUSTIFICATION_WORD_BOUND | TextServer::JUSTIFICATION_TRIM_EDGE_SPACES);
	l.char_offset = *r_char_offset;
	l.char_count = 0;