### 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)
} else {
			RS::get_singleton()->viewport_set_size(viewport, 0, 0);
		}
#ifndef _3D_DISABLED
	} 
#endif
	_update_global_transform();
	update_configuration_warnings();
	update_canvas_items();
	for (ViewportTexture *E : viewport_textures) {
		E->emit_changed();
	}
	emit_signal(SNAME("size_changed"));
}
Size2i Viewport::_get_size() const {
#ifndef _3D_DISABLED
	if (use_xr) {
		if (XRServer::get_singleton() != nullptr) {
			Ref<XRInterface> xr_interface = XRServer::get_singleton()->get_primary_interface();
			if (xr_interface.is_valid() && xr_interface->is_initialized()) {
				Size2 xr_size = xr_interface->get_render_target_size();
				return (Size2i)xr_size;
			}
		}
		return Size2i();
	}
#endif 
	return size;
}
Size2i Viewport::_get_size_2d_override() const {