### 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)
if (is_embedding_subwindows()) {
		return Transform2D();
	}
	SubViewportContainer *c = Object::cast_to<SubViewportContainer>(get_parent());
	if (!c) {
		return Viewport::get_screen_transform();
	}
	Transform2D container_transform;
	if (c->is_stretch_enabled()) {
		container_transform.scale(Vector2(c->get_stretch_shrink(), c->get_stretch_shrink()));
	}
	return c->get_screen_transform() * container_transform * Viewport::get_screen_transform();
}
void SubViewport::_notification(int p_what) {
	switch (p_what) {
		case NOTIFICATION_ENTER_TREE: {
			RS::get_singleton()->viewport_set_active(get_viewport_rid(), true);
		} break;
		case NOTIFICATION_EXIT_TREE: {
			RS::get_singleton()->viewport_set_active(get_viewport_rid(), false);
		} break;
	}
}
void SubViewport::_bind_methods() {