### 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)

};
	struct ItemTornado : public ItemFX {
		float radius = 1.0f;
		float frequency = 1.0f;
		Vector2 prev_off;
		ItemTornado() { type = ITEM_TORNADO; }
	};
	struct ItemRainbow : public ItemFX {
		float saturation = 0.8f;
		float value = 0.8f;
		float frequency = 1.0f;
		ItemRainbow() { type = ITEM_RAINBOW; }
	};
	struct ItemBGColor : public Item {
		Color color;
		ItemBGColor() { type = ITEM_BGCOLOR; }
	};
	struct ItemFGColor : public Item {
		Color color;
		ItemFGColor() { type = ITEM_FGCOLOR; }
	};
	struct ItemCustomFX : public ItemFX {
		Ref<CharFXTransform> char_fx_transform;
		Ref<RichTextEffect> custom_effect;
		ItemCustomFX() {
			type = ITEM_CUSTOMFX;
			char_fx_transform.instantiate();
		}
		virtual ~ItemCustomFX() {