@Override
    public TenantId getTenantId(NetworkId networkId) {
        VirtualNetwork virtualNetwork = getVirtualNetwork(networkId);
        checkNotNull(virtualNetwork, "The network does not exist.");
        return virtualNetwork.tenantId();
    }
@Test(expected = NullPointerException.class)
    public void testGetTenantIdForNullVirtualNetwork() {
        manager.getTenantId(null);
    }