Hi All,
I'm new to salesforce and started working on Trailhead's. I facing issue with the Trigger Challenge. Here is the triger code im writing 

trigger AccountAddressTrigger on Account (before insert) {
    for(Account a : Trigger.new){
        if(a.Match_Billing_Address__c && a.BillingPostalCode != null){
            a.ShippingPostalCode = a.BillingPostalCode;
        }
    }
    
}

but i'm getting the following error. Not sure where i'm doing wrong. can someone please help me here. Thanks in advance.

System.DmlException: Insert failed. First exception on row 0; first error: CANNOT_INSERT_UPDATE_ACTIVATE_ENTITY, TestTrigger: execution of AfterInsert caused by: System.NullPointerException: Attempt to de-reference a null object Trigger.TestTrigger: line 3, column 1: []