### 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 (world_2d.is_valid()) {
		return world_2d;
	} else if (parent) {
		return parent->find_world_2d();
	} else {
		return Ref<World2D>();
	}
}
void Viewport::_propagate_viewport_notification(Node *p_node, int p_what) {
	p_node->notification(p_what);
	for (int i = 0; i < p_node->get_child_count(); i++) {
		Node *c = p_node->get_child(i);
		if (Object::cast_to<Viewport>(c)) {
			continue;
		}
		_propagate_viewport_notification(c, p_what);
	}
}
Ref<World2D> Viewport::get_world_2d() const {