### 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 (!vp) {
		if (!vp_pending) {
			ERR_PRINT("Viewport Texture must be set to use it.");
		}
		return 0;
	}
	return vp->size.height;
}
Size2 ViewportTexture::get_size() const {
	if (!vp) {
		if (!vp_pending) {
			ERR_PRINT("Viewport Texture must be set to use it.");
		}
		return Size2();
	}
	return vp->size;
}
RID ViewportTexture::get_rid() const {
	if (proxy.is_null()) {
		proxy_ph = RS::get_singleton()->texture_2d_placeholder_create();
		proxy = RS::get_singleton()->texture_proxy_create(proxy_ph);
	}
	return proxy;
}
bool ViewportTexture::has_alpha() const {
	return false;
}
Ref<Image> ViewportTexture::get_image() const {