### 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)

if (_find_strikethrough(it)) {
				if (!st_started) {
					st_started = true;
					st_start = p_ofs + Vector2(off.x, off.y);
					st_color = font_color;
					st_color.a *= 0.5;
				}
			} else if (st_started) {
				st_started = false;
				float y_off = -TS->shaped_text_get_ascent(rid) + TS->shaped_text_get_size(rid).y / 2;
				float underline_width = TS->shaped_text_get_underline_thickness(rid) * theme_cache.base_scale;
				draw_line(st_start + Vector2(0, y_off), p_ofs + Vector2(off.x, off.y + y_off), st_color, underline_width);
			}
			
			ItemFade *fade = nullptr;
			Item *fade_item = it;
			while (fade_item) {
				if (fade_item->type == ITEM_FADE) {
					fade = static_cast<ItemFade *>(fade_item);
					break;
				}
				fade_item = fade_item->parent;
			}
			Vector<ItemFX *> fx_stack;
			_fetch_item_fx_stack(it, fx_stack);
			bool custom_fx_ok = true;
			Point2 fx_offset = Vector2(glyphs[i].x_off, glyphs[i].y_off);
			RID frid = glyphs[i].font_rid;
			uint32_t gl = glyphs[i].index;
			uint16_t gl_fl = glyphs[i].flags;