### Godot version

v4.0 rc6

### System information

Windows 10

### Issue description

When using a ViewportTexture from a SubViewport on a TextureRect, it throws errors after running the scene once and then switching scenes in the editor.
![Pasted image 20230227212626](https://user-images.githubusercontent.com/44676282/221680047-984fa03e-33d1-4856-9a7f-1700b28423cf.png)


### Steps to reproduce

Run the canvas-scene once.
Switch to another scene in the editor.
Switch back to the canvas-scene

### Minimal reproduction project

[ViewportTextureError.zip](https://github.com/godotengine/godot/files/10844099/ViewportTextureError.zip)

if (p_node != this) {
		Window *w = Object::cast_to<Window>(p_node);
		if (w && (!w->is_inside_tree() || !w->is_embedded())) {
			return;
		}
		CanvasItem *ci = Object::cast_to<CanvasItem>(p_node);
		if (ci) {
			ci->queue_redraw();
		}
	}
	int cc = p_node->get_child_count();
	for (int i = 0; i < cc; i++) {
		_update_canvas_items(p_node->get_child(i));
	}
}
Ref<ViewportTexture> Viewport::get_texture() const {
	ERR_READ_THREAD_GUARD_V(Ref<ViewportTexture>());
	return default_texture;
}
void Viewport::set_positional_shadow_atlas_size(int p_size) {
	ERR_MAIN_THREAD_GUARD;
	positional_shadow_atlas_size = p_size;
	RS::get_singleton()->viewport_set_positional_shadow_atlas_size(viewport, p_size, positional_shadow_atlas_16_bits);
}
int Viewport::get_positional_shadow_atlas_size() const {