### 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)

#ifndef _3D_DISABLED
	if (!use_xr) {
#endif
		if (p_allocated) {
			RS::get_singleton()->viewport_set_size(viewport, size.width, size.height);
		} else {
			RS::get_singleton()->viewport_set_size(viewport, 0, 0);
		}
#ifndef _3D_DISABLED
	} 
#endif
	_update_global_transform();
	update_configuration_warnings();
	update_canvas_items();
	for (ViewportTexture *E : viewport_textures) {
		E->emit_changed();
	}
	emit_signal(SNAME("size_changed"));
	Rect2i limit = get_visible_rect();
	for (int i = 0; i < gui.sub_windows.size(); ++i) {
		Window *sw = gui.sub_windows[i].window;
		Rect2i rect = Rect2i(sw->position, sw->size);
		Rect2i new_rect = sw->fit_rect_in_parent(rect, limit);
		if (new_rect != rect) {
			sw->set_position(new_rect.position);
			sw->set_size(new_rect.size);
		}
	}
}
Size2i Viewport::_get_size() const {