Hi
I have created an apex trigger to update contact address when the account address is changed. The code worked fine for already created accounts, but when I have created a new account the following error was thrown and I was not able to save the account. Please help me.

Error: updateContact: execution of AfterInsert caused by: System.NullPointerException: Attempt to de-reference a null object Trigger.updateContact: line 33, column 1​
 

trigger updateContact on Account (after insert, after update) {
    List<Contact> contactsForUpsert = New List<Contact>();

    Map<Id, Contact> contactsByAccountId = new Map<Id, Contact>();
    List<Contact> contacts = [SELECT Id,
                                     AccountId,
                                     mailingstreet,
                                     mailingcity,
                                     mailingstate,
                                     mailingpostalcode
                            FROM Contact WHERE AccountId IN: Trigger.newMap.keySet()];
    for (Contact cont: contacts) {
        contactsByAccountId.put(cont.accountId, cont);
    }

    for (Account account: Trigger.new) {

        Account old;
        if (Trigger.isUpdate) {
            old = Trigger.oldMap.get(account.Id);
        }

        Boolean isShippingAddressChangedOrNew = Trigger.isInsert ? true :
                                                (   account.ShippingStreet != old.ShippingStreet
                                                 || account.ShippingCity != old.ShippingCity
                                                 || account.ShippingState != old.ShippingState
                                                 || account.ShippingPostalCode != old.ShippingPostalCode
                                                 || account.ShippingCountry != old.ShippingCountry) ? true : false;

        if (isShippingAddressChangedOrNew) {
            Contact relatedContact = contactsByAccountId.get(account.Id);
           
            relatedContact.mailingstreet     = account.Shippingstreet;
            relatedContact.mailingcity       = account.Shippingcity;
            relatedContact.mailingstate      = account.Shippingstate;
            relatedContact.mailingpostalcode = account.shippingpostalcode;
            contactsForUpsert.add(relatedContact);
        

    }

    try {
        upsert contactsForUpsert;
    } catch(Dmlexception e) {
        System.debug(LoggingLevel.ERROR, 'Contact insert from createopp.trigger has failed with message:' + e.getMessage());
    }
}
}