### 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)

ERR_MAIN_THREAD_GUARD;
	gui.canvas_sort_index = 0;
}
int Viewport::gui_get_canvas_sort_index() {
	ERR_MAIN_THREAD_GUARD_V(0);
	return gui.canvas_sort_index++;
}
void Viewport::gui_release_focus() {
	ERR_MAIN_THREAD_GUARD;
	if (gui.key_focus) {
		Control *f = gui.key_focus;
		gui.key_focus = nullptr;
		f->notification(Control::NOTIFICATION_FOCUS_EXIT, true);
		f->queue_redraw();
	}
}
Control *Viewport::gui_get_focus_owner() const {
	ERR_READ_THREAD_GUARD_V(nullptr);
	return gui.key_focus;
}
void Viewport::set_msaa_2d(MSAA p_msaa) {
	ERR_MAIN_THREAD_GUARD;
	ERR_FAIL_INDEX(p_msaa, MSAA_MAX);
	if (msaa_2d == p_msaa) {
		return;
	}
	msaa_2d = p_msaa;
	RS::get_singleton()->viewport_set_msaa_2d(viewport, RS::ViewportMSAA(p_msaa));
}
Viewport::MSAA Viewport::get_msaa_2d() const {