@DurgaPaavan,
Please find the createuser method below
 

global User createUser(Id portalId, Auth.UserData data){
        if(!canCreateUser(data)) {
            //Returning null or throwing an exception fails the SSO flow
            return null;
        }
        User impersonatorUser = verifyImpersonator(data);
        if(impersonatorUser != null) {
            return impersonatorUser;
        }
        // Check if User is already present with Google Id
        list<User> usrList = [select Id from User where Contact.Account.Account_Id__c = :data.username and Contact.Account.Account_Status__c = 'Active' ORDER BY lastmodifieddate desc limit 1];
        if(usrList != null && usrList.size() > 0) {
            return usrList[0];
        }
        else {
            System.debug('IDX = ' + data.username);
            //The user is authorized, so create their Salesforce user
            list<Contact> con = [select Id, AccountId, Account.Account_Id_Format__c, Account.Account_Request__c from Contact 
                where Account.Account_Id__c =:data.username and Account.Account_Status__c = 'Active' limit 1];
            if(con != null && con.size() > 0) {
                User u = new User();
                Profile p = [SELECT Id FROM profile WHERE name='Paid Account'];


                String emailUser = data.email.split('@')[0];
                String emailDomain = data.email.split('@')[1];

                //TODO: Customize the username. Also check that the username doesn't already exist and
                //possibly ensure there are enough org licenses to create a user. Must be 80 characters or less.
                u.username = data.email;
                u.email = data.email;
                u.lastName = data.lastName;
                u.firstName = data.firstName;
                String alias = emailUser;
                //Alias must be 8 characters or less
                if(alias.length() > 8) {
                    alias = alias.substring(0, 8);
                }
                u.alias = alias;
                u.languagelocalekey = UserInfo.getLocale();
                u.localesidkey = UserInfo.getLocale();
                u.emailEncodingKey = 'UTF-8';
                u.timeZoneSidKey = 'America/Phoenix';
                u.profileId = p.Id;
                u.ContactId = con[0].Id;
                u.IsActive = true;
                u.CompanyName = 'Custom User';
                u.UserPreferencesHideS1BrowserUI = true;

                try {
                    if(con[0].AccountId != null && con[0].Account.Account_Request__c != null) {
                        String jsonStr = con[0].Account.Account_Request__c;
                        PD_AccountUserWS.PD_AccountUserRequest pupr = (PD_AccountUserWS.PD_AccountUserRequest) JSON.deserialize (jsonStr,
                        PD_AccountUserWS.PD_AccountUserRequest.class);

                        if(pupr != null) {
                            if(pupr.locale != null) {
                                // Validate if stamped locale is valid, before trying to set it
                                Schema.DescribeFieldResult fieldResult = User.localesidkey.getDescribe();
                                List<Schema.PicklistEntry> ple = fieldResult.getPicklistValues();                                   
                                // If the locale is in the picklist, then set it
                                for (Schema.PicklistEntry pli : ple){
                                    if (pupr.locale.equals(pli.getValue())){
                                        u.localesidkey = pupr.locale;
                                    }
                                }
                            }
                            if(pupr.timezone != null) {
                                u.timeZoneSidKey = pupr.timezone;
                            }
                            if(pupr.language != null) {
                                // Validate if stamped language is valid, before trying to set it
                                Schema.DescribeFieldResult fieldResult = User.languagelocalekey.getDescribe();
                                List<Schema.PicklistEntry> ple = fieldResult.getPicklistValues();                                   

                                // Blindly try first 2 chars of language
                                String lng = pupr.language.substring(0,2);
                                for (Schema.PicklistEntry pli : ple){
                                    if (lng.equals(pli.getValue())){
                                        u.languagelocalekey = lng;
                                    }
                                }                                                                                                                   

                                // If the Language is in the picklist, then set it, potentially overriding the previous loop
                                // Handles cases like en_US, en_UK, en_CA, fr_CA, pt_BR, es_MX
                                for (Schema.PicklistEntry pli : ple){
                                    if (pupr.language.equals(pli.getValue())){
                                        u.languagelocalekey = pupr.language;
                                    }
                                }                                                                                 
                                if (!u.languagelocalekey.equals(pupr.language) && !u.languagelocalekey.equals(lng)){
                                    Key_Value_List__c tmp = Key_Value_List__c.getValues('PD_LanguageOverride_' + lng);
                                    if (tmp != null){
                                        String overrideLang = tmp.Value__c;
                                        for (Schema.PicklistEntry pli : ple){
                                            if (overrideLang.equals(pli.getValue())){
                                                u.languagelocalekey = overrideLang;
                                            }
                                        }                                                       
                                    }
                                }

                                // else do nothing, fallback to English                          
                            }
                        }
                    }
                } catch (Exception ex) {}

                insert u;

                String tmpStr = 'GROUP_' + u.Id;
                Group grpDL;
                list<Group> extGrp = [select Id from Group where DeveloperName=:tmpStr and Type='Regular' and Name like 'My Account %' limit 1];
                if(extGrp != null && extGrp.size() > 0) {
                    grpDL = extGrp[0];
                } 
                else {
                    grpDL = new Group(DeveloperName='GROUP_' + u.Id, 
                    DoesIncludeBosses=false, 
                    DoesSendEmailToMembers=false,
                    Name='My Account (' + con[0].Account.Account_Id_Format__c + ')', 
                    Type='Regular');
                    insert grpDL;
                }



                // Insert Chatter Group Members and User Shares
                if(groupDLMembers.size() > 0) {
                    Database.Saveresult[] sr = Database.insert(groupDLMembers, false);
                }

                chatterSetup(u.Id, grpDL.Id);
                return u;
            }
            else {
                return null;
            }
        }
    }

I am recieving null from this method in  the code. THis is working fine in real time scenario whereas in test class, it is returning null.
Also, using userinfo.getuserid() returns my id. But here I am inserting communtiy user. Is that the reason for null pointer exception. Please suggest