### 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)

word_start = -1;
				word_length = 0;
			}
		} else if (breaks.has(i)) {
			// End current word, no space.
			if (word_length > 0) {
				line_end = i;
				word_start = i + 1;
				word_length = 0;
			}
			if (p_chars_per_line <= 0) {
				ret.push_back(line_start);
				);
				line_start = word_start;
				line_end = line_start;
			}
		} else {
			if (word_start == -1) {
				word_start = i;
				if (p_chars_per_line <= 0) {
					ret.push_back(line_start);
					);
					line_start = word_start;
					line_end = line_start;
				}
			}
			word_length += 1;
			if (p_chars_per_line > 0) {
				if (word_length > p_chars_per_line) {
					// Word too long: wrap before current character.