I have made the changes suggested by you, though still its returniung null

public PageReference registerUser() {
        System.debug('--method called---');
           // it's okay if password is null - we'll send the user a random password in that case
        

        String profileId = '00e28000000x7rk'; // To be filled in by customer.
        String roleEnum = 'Customer User'; // To be filled in by customer.
        String accountId = '001280000093OaC'; // To be filled in by customer.
        
        String userName = email;

        User u = new User();
        u.Username = 'naveen@portal.com';
        u.Email = 'naveen.sfdc21@gmail.com';
        u.FirstName = 'Naveen';
        u.LastName = 'Kakrla';
        u.CommunityNickname = 'naveen.sf21';
        u.ProfileId = profileId;
        
        String userId;

       
        try {
            System.debug('--in try---');
            System.debug('---user---'+u);
            userId = Site.createExternalUser(u, accountId, password);
            System.debug(userId);
        } catch(Site.ExternalUserCreateException ex) {
            System.debug('--catch---');
            List<String> errors = ex.getDisplayMessages();
            for (String error : errors)  {
                ApexPages.addMessage(new ApexPages.Message(ApexPages.Severity.ERROR, error));
            }
            System.debug(ex.getMessage());
        }
        
        if (userId != null) { 
            if (password != null && password.length() > 1) {
                System.debug('--in if password---');
                return Site.login(userName, password, ApexPages.currentPage().getParameters().get('startURL'));
            }
            else {
                System.debug('--in else password---');
                PageReference page = System.Page.CommunitiesSelfRegConfirm;
                page.setRedirect(true);
                return page;
            }
        }
        return null;
    }