### 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_READ_THREAD_GUARD_V(Size2());
	return _get_size();
}
void SubViewport::set_size_2d_override(const Size2i &p_size) {
	ERR_MAIN_THREAD_GUARD;
	_set_size(_get_size(), p_size, true);
}
Size2i SubViewport::get_size_2d_override() const {
	ERR_READ_THREAD_GUARD_V(Size2i());
	return _get_size_2d_override();
}
void SubViewport::set_size_2d_override_stretch(bool p_enable) {
	ERR_MAIN_THREAD_GUARD;
	if (p_enable == size_2d_override_stretch) {
		return;
	}
	size_2d_override_stretch = p_enable;
	_set_size(_get_size(), _get_size_2d_override(), true);
}
bool SubViewport::is_size_2d_override_stretch_enabled() const {