i have  designed a apex class on custom object where it checks the duplicate value for Specific recordtype, it is invoked by a trigger handler, i am getting this error - 

QuotaTrigger: execution of BeforeInsert caused by: System.NullPointerException: Attempt to de-reference a null object Class.QuotaTriggerHandler.dupQuotaError: line 5, column 1 Trigger.QuotaTrigger: line 3, column 1

I tried Googling the error but still   not able to Resolve the error, Any suggestions please

QuotaTrigger
 

trigger QuotaTrigger on Quota__c (before insert, before update) {

    QuotaTriggerHandler.dupQuotaError(Trigger.new);
    
}

QuotaTriggerHandler
 

public class QuotaTriggerHandler {
    
    public static void dupQuotaError (List <Quota__c> newTrigCon) {
        
        Id recordTypeId = Schema.SObjectType.Quota__c.getRecordTypeInfosByDeveloperName().get('Inside Sales Target(IS)').getRecordTypeId();
        
        //List of all Contact Records currently in the database    
        List <Quota__c> allCon = [SELECT ID, Name, Assignedto__c, Quater_Year__c, RecordTypeId, Month__c FROM Quota__c where RecordTypeId=:recordTypeId];
        
        //Iterate through new potential records
        for (Quota__c oneConTrig : newTrigCon) {
            
            //Needed to iterate through all existing records which is contained in the allCon List
            for (Integer i = 0; i < allCon.size(); i++)
                
                //If all conditions are met, there is a duplicate and thus, returns an error.
                if (oneContrig.Assignedto__c == allCon[i].Assignedto__c 
                    && oneContrig.Quater_Year__c == allCon[i].Quater_Year__c 
                    && oneConTrig.Month__c == allCon[i].Month__c
                    && oneConTrig.RecordTypeId == recordTypeId) {
                        
                        oneConTrig.addError('Duplicate Quota Detected! This record already exists.');
                        
                    }
        }      
    }
}

How do i resove this issue