### 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 (font_it->font_size > 0) {
				font_size = font_it->font_size;
			}
		}
		ItemFontSize *font_size_it = _find_font_size(l.from);
		if (font_size_it && font_size_it->font_size > 0) {
			font_size = font_size_it->font_size;
		}
		if (rtl) {
			float offx = 0.0f;
			if (!lrtl && p_frame == main) { 
				offx -= scroll_w;
			}
			font->draw_string(ci, p_ofs + Vector2(p_width - l.offset.x + offx, l.text_buf->get_line_ascent(0)), " " + prefix, HORIZONTAL_ALIGNMENT_LEFT, l.offset.x, font_size, _find_color(l.from, p_base_color));
		} else {
			float offx = 0.0f;
			if (lrtl && p_frame == main) { 
				offx += scroll_w;
			}
			font->draw_string(ci, p_ofs + Vector2(offx, l.text_buf->get_line_ascent(0)), prefix + " ", HORIZONTAL_ALIGNMENT_RIGHT, l.offset.x, font_size, _find_color(l.from, p_base_color));
		}
	}
	
	int dc_lines = l.text_buf->get_dropcap_lines();
	float h_off = l.text_buf->get_dropcap_size().x;
	if (l.dc_ol_size > 0) {
		l.text_buf->draw_dropcap_outline(ci, p_ofs + ((rtl) ? Vector2() : Vector2(l.offset.x, 0)), l.dc_ol_size, l.dc_ol_color);
	}
	l.text_buf->draw_dropcap(ci, p_ofs + ((rtl) ? Vector2() : Vector2(l.offset.x, 0)), l.dc_color);
	int line_count = 0;