### Godot version

v4.0.beta.custom_build [0a9e6e478]

### System information

Ubuntu, Nvidia, Vulkan

### Issue description

When a Camera2D is freed - either explicitly or by being a child of a scene that is being switched away from via a method like `get_tree().change_scene_to_file()` - then this camera somehow stays active instead of Godot properly switching to no-camera-mode.

`get_viewport().get_camera_2d()` then returns all sorts of funny things from `<Freed Object>`  (it's supposed to return `null`) to arbitrary nodes of the current scene (not cameras).

This can lead to crashes.

### Steps to reproduce

checkout the MRP.

note that the "Go to scene 2" button switches places after pressing it.
that is because a camera is still active that does not exist in the current tree.

restart it a couple of times and spam the "Go to scene 2" button.
notice the garbage output.

### Minimal reproduction project

[godot_4_camera_bug.zip](https://github.com/godotengine/godot/files/10560455/godot_4_camera_bug.zip)
return _get_size();
}
void SubViewport::set_size_2d_override(const Size2i &p_size) {
	_set_size(_get_size(), p_size, Rect2i(), true);
}
Size2i SubViewport::get_size_2d_override() const {
	return _get_size_2d_override();
}
void SubViewport::set_size_2d_override_stretch(bool p_enable) {
	if (p_enable == size_2d_override_stretch) {
		return;
	}
	size_2d_override_stretch = p_enable;
	_set_size(_get_size(), _get_size_2d_override(), Rect2i(), true);
}
bool SubViewport::is_size_2d_override_stretch_enabled() const {
	return size_2d_override_stretch;
}
void SubViewport::set_update_mode(UpdateMode p_mode) {