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: []

create an Apex class that inserts a new account named after an incoming parameter. If the account is successfully inserted, the method should return the account record. If a DML exception occurs, the method should return null?
The Apex class must be called 'AccountHandler' and be in the public scope.
The Apex class must have a public static method called 'insertNewAccount'.
The 'insertNewAccount' method must accept an incoming string as a parameter, name the account after the parameter, insert it into the system and then return the account record.
The 'insertNewAccount' method must also accept an empty string, catch the failed DML and return null.