### 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 follow_smoothing_enabled;
}
void Camera2D::set_custom_viewport(Node *p_viewport) {
	ERR_FAIL_NULL(p_viewport);
	if (is_inside_tree()) {
		remove_from_group(group_name);
		remove_from_group(canvas_group_name);
	}
	custom_viewport = Object::cast_to<Viewport>(p_viewport);
	if (custom_viewport) {
		custom_viewport_id = custom_viewport->get_instance_id();
	} else {
		custom_viewport_id = ObjectID();
	}
	if (is_inside_tree()) {
		if (custom_viewport) {
			viewport = custom_viewport;
		} else {
			viewport = get_viewport();
		}
		RID vp = viewport->get_viewport_rid();
		group_name = "__cameras_" + itos(vp.get_id());
		canvas_group_name = "__cameras_c" + itos(canvas.get_id());
		add_to_group(group_name);
		add_to_group(canvas_group_name);
	}
}
Node *Camera2D::get_custom_viewport() const {