Not able to assign group and membership programmatically in JPP 6

Issue

I have created a new Membership using the classic portal. The name of the new membership is "foo". Now, while registration of a user, I am trying to assign this membership to the user. My code for the same is:

    OrganizationService organizationService = (OrganizationService) PortalContainer
                .getInstance().getComponentInstanceOfType(OrganizationService.class);

    User newUser = new UserImpl();
    newUser.setFirstName(userBean.getFirstName());
    newUser.setLastName(userBean.getLastName());
    newUser.setUserName(userBean.getUserName());
    newUser.setEmail(userBean.getEmail());
    newUser.setPassword(userBean.getPassword());
    organizationService.getUserHandler().createUser(newUser, Constants.TRUE);

    Group newGroup=new GroupImpl();
    newGroup.setGroupName("Users");
    newGroup.setDescription("Users");
    newGroup.setLabel("Users");
    MembershipType newMembershipType=new MembershipTypeImpl();
    newMembershipType.setName("foo");
    newMembershipType.setDescription("foo membership type");
    organizationService.getMembershipHandler().linkMembership(newUser, newGroup, newMembershipType, Constants.TRUE);

But this code fails with a NullPointerException on the last line.