### 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)

Item *it = p_frame;
	while (it) {
		ItemFX *ifx = nullptr;
		if (it->type == ITEM_CUSTOMFX || it->type == ITEM_SHAKE || it->type == ITEM_WAVE || it->type == ITEM_TORNADO || it->type == ITEM_RAINBOW) {
			ifx = static_cast<ItemFX *>(it);
		}
		if (!ifx) {
			it = _get_next_item(it, true);
			continue;
		}
		ifx->elapsed_time += p_delta_time;
		ItemShake *shake = nullptr;
		if (it->type == ITEM_SHAKE) {
			shake = static_cast<ItemShake *>(it);
		}
		if (shake) {
			bool cycle = (shake->elapsed_time > (1.0f / shake->rate));
			if (cycle) {
				shake->elapsed_time -= (1.0f / shake->rate);
				shake->reroll_random();
			}
		}
		it = _get_next_item(it, true);
	}
}
int RichTextLabel::_find_first_line(int p_from, int p_to, int p_vofs) const {