### 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(nullptr);
	return camera_3d;
}
void Viewport::_camera_3d_transform_changed_notify() {
}
void Viewport::_camera_3d_set(Camera3D *p_camera) {
	if (camera_3d == p_camera) {
		return;
	}
	if (camera_3d) {
		camera_3d->notification(Camera3D::NOTIFICATION_LOST_CURRENT);
	}
	camera_3d = p_camera;
	if (!camera_3d_override) {
		if (camera_3d) {
			RenderingServer::get_singleton()->viewport_attach_camera(viewport, camera_3d->get_camera());
		} else {
			RenderingServer::get_singleton()->viewport_attach_camera(viewport, RID());
		}
	}
	if (camera_3d) {
		camera_3d->notification(Camera3D::NOTIFICATION_BECAME_CURRENT);
	}
	_update_audio_listener_3d();
	_camera_3d_transform_changed_notify();
}
bool Viewport::_camera_3d_add(Camera3D *p_camera) {