### 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)

List<Item *>::Element *E = p_reverse_search ? table->subitems.back() : table->subitems.front();
				if (_search_table(table, E, p_string, p_reverse_search)) {
					return true;
				}
			} break;
			default:
				break;
		}
	}
	int sp = -1;
	if (p_reverse_search) {
		sp = txt.rfindn(p_string, p_char_idx);
	} else {
		sp = txt.findn(p_string, p_char_idx);
	}
	if (sp != -1) {
		selection.from_frame = p_frame;
		selection.from_line = p_line;
		selection.from_item = _get_item_at_pos(l.from, it_to, sp);
		selection.from_char = sp;
		selection.to_frame = p_frame;
		selection.to_line = p_line;
		selection.to_item = _get_item_at_pos(l.from, it_to, sp + p_string.length());
		selection.to_char = sp + p_string.length();
		selection.active = true;
		return true;
	}
	return false;
}
bool RichTextLabel::search(const String &p_string, bool p_from_selection, bool p_search_previous) {