### 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 (fade) {
				float faded_visibility = 1.0f;
				if (glyphs[i].start >= fade->starting_index) {
					faded_visibility -= (float)(glyphs[i].start - fade->starting_index) / (float)fade->length;
					faded_visibility = faded_visibility < 0.0f ? 0.0f : faded_visibility;
				}
				font_outline_color.a = faded_visibility;
				font_shadow_color.a = faded_visibility;
			}
			bool txt_visible = (font_outline_color.a != 0) || (font_shadow_color.a != 0);
			for (int j = 0; j < fx_stack.size(); j++) {
				ItemFX *item_fx = fx_stack[j];
				bool cn = cprev_cluster || (cprev_conn && item_fx->connected);
				if (item_fx->type == ITEM_CUSTOMFX && custom_fx_ok) {
					ItemCustomFX *item_custom = static_cast<ItemCustomFX *>(item_fx);
					Ref<CharFXTransform> charfx = item_custom->char_fx_transform;
					Ref<RichTextEffect> custom_effect = item_custom->custom_effect;
					if (!custom_effect.is_null()) {
						charfx->elapsed_time = item_custom->elapsed_time;
						charfx->range = Vector2i(l.char_offset + glyphs[i].start, l.char_offset + glyphs[i].end);
						charfx->relative_index = l.char_offset + glyphs[i].start - item_fx->char_ofs;
						charfx->visibility = txt_visible;
						charfx->outline = true;
						charfx->font = frid;
						charfx->glyph_index = gl;
						charfx->glyph_flags = gl_fl;
						charfx->glyph_count = gl_cn;
						charfx->offset = fx_offset;