@Override
    public Iterable<Device> getDevices() {
        return manager.getVirtualDevices(
                this.networkId).stream().collect(Collectors.toSet());
    }
@Test(expected = NullPointerException.class)
    public void testGetDeviceByNullType() {
        manager.registerTenantId(TenantId.tenantId(tenantIdValue1));
        VirtualNetwork virtualNetwork = manager.createVirtualNetwork(TenantId.tenantId(tenantIdValue1));
        DeviceService deviceService = manager.get(virtualNetwork.id(), DeviceService.class);

        // test the getDevices() method with null type value.
        deviceService.getDevices(null);
    }