### 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 (sep == -1) {
						width = bbcode_value.to_int();
					} else {
						width = bbcode_value.substr(0, sep).to_int();
						height = bbcode_value.substr(sep + 1).to_int();
					}
				} else {
					OptionMap::Iterator width_option = bbcode_options.find("width");
					if (width_option) {
						width = width_option->value.to_int();
					}
					OptionMap::Iterator height_option = bbcode_options.find("height");
					if (height_option) {
						height = height_option->value.to_int();
					}
				}
				add_image(texture, width, height, color, (InlineAlignment)alignment, region);
			}
			pos = end;
			tag_stack.push_front(bbcode_name);
		} else if (tag.begins_with("color=")) {
			String color_str = tag.substr(6, tag.length());
			Color color = Color::from_string(color_str, theme_cache.default_color);
			push_color(color);
			pos = brk_end + 1;
			tag_stack.push_front("color");
		} else if (tag.begins_with("outline_color=")) {
			String color_str = tag.substr(14, tag.length());
			Color color = Color::from_string(color_str, theme_cache.default_color);
			push_outline_color(color);