### Godot version

4.2 eca6f0e

### System information

Windows 10 x64

### Issue description


https://github.com/godotengine/godot/assets/2223172/a2796263-4e2e-4b14-a080-b3ad034cc141



### Steps to reproduce

1. Add SubViewport
2. Add Sprite2D
3. Set spirte's texture to ViewportTexture
4. Create new scene
5. Instance the new scene inside the scene with viewport
6. Make sure both scenes are opened
7. Go to instance scene and save it
8. Go to viewport scene

You will see error spam. Reloading the scene fixes it.

### Minimal reproduction project

[vip.zip](https://github.com/godotengine/godot/files/12292386/vip.zip)

ERR_READ_THREAD_GUARD_V(SDF_SCALE_100_PERCENT);
	return sdf_scale;
}
Transform2D Viewport::get_screen_transform() const {
	ERR_READ_THREAD_GUARD_V(Transform2D());
	return get_screen_transform_internal();
}
Transform2D Viewport::get_screen_transform_internal(bool p_absolute_position) const {
	ERR_READ_THREAD_GUARD_V(Transform2D());
	return get_final_transform();
}
void Viewport::update_mouse_cursor_state() {
	
	Window *base_window = get_base_window();
	if (base_window) {
		base_window->update_mouse_cursor_state();
	}
}
void Viewport::set_canvas_cull_mask(uint32_t p_canvas_cull_mask) {
	ERR_MAIN_THREAD_GUARD;
	canvas_cull_mask = p_canvas_cull_mask;
	RenderingServer::get_singleton()->viewport_set_canvas_cull_mask(viewport, canvas_cull_mask);
}
uint32_t Viewport::get_canvas_cull_mask() const {
	ERR_READ_THREAD_GUARD_V(0);
	return canvas_cull_mask;
}
void Viewport::set_canvas_cull_mask_bit(uint32_t p_layer, bool p_enable) {