I have done more modification and now null pointer at line 65


trigger updateAccCorefrmProperty_final on Property__c (after insert, after update, after delete) {


      Set<Id> accid = new Set<Id> ();
      Set<Id> conids= new Set<Id> ();
        Set<Id> conids2= new Set<Id> ();
      Map<id,Id> mpConAccIds= new Map<id,Id>();        
      Map<id,List<Contact>> mpConAccIds2= new Map<id,List<Contact>>();        
      Map<id,List<Property__c>> mpConProp = new  Map<id,List<Property__c>>();     
      Set<String>  PropCores= new Set<String>();
      Map<Id,Set<String>> mpCores = new Map<id,Set<String>> ();
      List<Account> acco = new List<Account>();
      set<id>  propids= new set<id> ();
           
    if(Trigger.IsInsert || Trigger.IsUpdate ) {
       for(Property__c prp: [select id,core__c ,Contact__c,Contact__r.Accountid,Contact__r.Account.core__c from Property__c where id in : Trigger.new] )
       {
        if(prp.Contact__c!=null && prp.core__c !=null)
        accid.add(prp.Contact__r.Accountid);
        conids2.add(prp.Contact__c);
        
        }
       }
 
    if(Trigger.IsDelete ){
     
         for(Property__c prp :Trigger.old){
             if(prp.Contact__c!=null && prp.core__c !=null)
             propids.add(prp.Contact__c);        
         }
         
         for(Contact con: [select id, AccountId from Contact where id in : propids] )
         {  
              
            accid.add(con.Accountid);
         }    
     }    
    
       for(Account acc: [select id,(select id,AccountId from Contacts) from Account where id in : accid]){
       
          mpConAccIds2.put(acc.id,acc.Contacts);
       
              for(Contact con : acc.Contacts){
                  conids.add(con.id);
               }      
       }
       
       
       for(Contact cont: [select id,(select id,Core__c from Properties__r where Core__c !=null) from Contact where id in : conids2]){
       
       if(cont.Properties__r !=null)
           mpConProp.put(cont.id,cont.Properties__r);
           
           }

          
          
           for(id acc: mpConAccIds2.keyset()){
               PropCores.clear();
               List<Contact>  lts =mpConAccIds2.get(acc);
               if(lts.size()>0){
               for(Contact cons : mpConAccIds2.get(acc) ){
                   
                   List<Property__c >  ltspp =mpConProp.get(cons.id);
                       if(ltspp.size()>0){
               
                   for(Property__c prop: mpConProp.get(cons.id) ){
                       if (prop.core__c != null){
                           PropCores.add(prop.core__c);                   
                       }
                    }}
                   
                   }    
               }
                     mpCores.put(acc,PropCores);           
               
               }
               
      String new_propCore;
       
       for(Account acc : [select id,core__c from Account where id in :mpCores.keyset()]){
           for(String str: mpCores.get(acc.id)){
               if(new_propCore!=null)
                   new_propCore+= ','+str;
               else{
               new_propCore=str;
               }    
           }
               acc.core__c =new_propCore ;
               acco.add(acc);
       }
       
 
       if(acco.size()>0)
       update acco;
        
        

}