### 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)
canvas_layers.erase(p_canvas_layer);
}
void Viewport::set_transparent_background(bool p_enable) {
	transparent_bg = p_enable;
	RS::get_singleton()->viewport_set_transparent_background(viewport, p_enable);
}
bool Viewport::has_transparent_background() const {
	return transparent_bg;
}
void Viewport::set_world_2d(const Ref<World2D> &p_world_2d) {
	if (world_2d == p_world_2d) {
		return;
	}
	if (is_inside_tree()) {
		RenderingServer::get_singleton()->viewport_remove_canvas(viewport, current_canvas);
	}
	if (p_world_2d.is_valid()) {
		world_2d = p_world_2d;
	} else {
		WARN_PRINT("Invalid world_2d");
		world_2d = Ref<World2D>(memnew(World2D));
	}
	_update_audio_listener_2d();
	if (is_inside_tree()) {
		current_canvas = find_world_2d()->get_canvas();
		RenderingServer::get_singleton()->viewport_attach_canvas(viewport, current_canvas);
	}
}
Ref<World2D> Viewport::find_world_2d() const {