### 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)

ItemFontSize *font_size_it = _find_font_size(it);
				if (font_size_it && font_size_it->font_size > 0) {
					font_size = font_size_it->font_size;
				}
				String lang = _find_language(it);
				String tx = t->text;
				if (visible_chars_behavior == TextServer::VC_CHARS_BEFORE_SHAPING && visible_characters >= 0 && remaining_characters >= 0) {
					tx = tx.substr(0, remaining_characters);
				}
				remaining_characters -= tx.length();
				l.text_buf->add_string(tx, font, font_size, lang, (uint64_t)it);
				txt += tx;
				l.char_count += tx.length();
			} break;
			case ITEM_IMAGE: {
				ItemImage *img = static_cast<ItemImage *>(it);
				l.text_buf->add_object((uint64_t)it, img->size, img->inline_align, 1);
				txt += String::chr(0xfffc);
				l.char_count++;
				remaining_characters--;
			} break;
			case ITEM_TABLE: {
				ItemTable *table = static_cast<ItemTable *>(it);
				int col_count = table->columns.size();
				int t_char_count = 0;
				
				for (int i = 0; i < col_count; i++) {
					table->columns[i].min_width = 0;
					table->columns[i].max_width = 0;
					table->columns[i].width = 0;