The following line is causing the issue  .. becasue you use the activityIds in the method arguments  and not initlizeed

  public static void createCommunicatOHistory(List<id> activityIds, String message, Datetime scheduleDateTime){


change it as below 

 

public class SMSHelper {
    public static List<Task> lstTask;
    public static List<Communicat_O_activity__c> lstComActivity;
    public SMSHelper(){
        lstTask = new List<Task>();
        lstComActivity = new List<Communicat_O_activity__c>();
    } 
    
    
    public static void createActivities(List<Id> recordIdSet ,String message, Datetime scheduleDateTime){
        SMS_Configuration__c smsConfig = SMS_Configuration__c.getOrgDefaults();
        String status = scheduleDateTime == null?'Completed':'Scheduled';
        List<Id> activityIds = new List<Id>();
        if(smsConfig.Use_Salesforce_Activity__c){
            
            createSalesforceActivities(recordIdSet, message, scheduleDateTime, status);
        }
        else{
            createCommunicatoActivities(recordIdSet, message, scheduleDateTime, status);
        }
        SMSHelper.createCommunicatOHistory(activityIds, message, scheduleDateTime);
        
        if(status == 'Completed'){
            
        } 
        // SMSHelper.createCommunicatOHistory(List<id> activityIds);
    }
    
    public static void createCommunicatOHistory(List<id> activityIds, String message, Datetime scheduleDateTime){
        system.debug('lsttask' + lstTask);
        system.debug('comactivity' + lstComActivity);
		activityIds = new List<Id>();
        if(lstTask.size()>0){
            for(Task t : lstTask){
                activityIds.add(t.id);
            }
        }
        if(lstComActivity.size()>0){
            for(Communicat_O_activity__c com : lstComActivity){
                activityIds.add(com.id);
            }
        }
        Communicato_History__c comHistory = new Communicato_History__c();
        comHistory.Message__c = message;
        comHistory.Schedule_Date_Time__c = scheduleDateTime;
        comHistory.Activity_Ids__c = String.valueOf(activityIds);
        insert comHistory;
    }
    
    public static void createSalesforceActivities(List<Id> recordIdSet ,String message, Datetime scheduleDateTime, String status){
        List<Task> lstTask = new List<Task>();
        
        for(String id: recordIdSet){
            Task taskObj = new Task();
            taskObj.ActivityDate = Date.valueOf(scheduleDateTime);
            taskObj.Status = status;
            taskObj.Description = message;
            taskObj.WhoId = id;
            
            lstTask.add(taskObj); 
        }
        insert lstTask;
        
        
        // Map<Id, Task> TaskMap = tnew Map<Id, Task>(lstTask);
    }
    
    public static void createCommunicatoActivities(List<Id> recordIdSet ,String message, Datetime scheduleDateTime, String status){
        List<Communicat_O_activity__c> lstComActivity = new List<Communicat_O_activity__c>();
        
        for(String id: recordIdSet){
            Communicat_O_activity__c comObj = new Communicat_O_activity__c();
            
            comObj.Schedule_Date_Time__c = scheduleDateTime;
            comObj.Status__c = status;
            comObj.Message__c = message;
            comObj.Related_To__c = id;
            
            lstComActivity.add(comObj); 
        }
        
        insert lstComActivity;
        
    }
    
}