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: []

hi
i am new to salesforce platform.
Install a simple Apex class, write unit tests that achieve 100% code coverage for the class, and run your Apex tests.The Apex class to test is called 'VerifyDate', and the code is available here. Copy and paste this class into your Developer Edition via the Developer Console.
'VerifyDate' is a class which tests if a date is within a proper range, and if not will return a date that occurs at the end of the month within the range.
The unit tests must be in a separate test class called 'TestVerifyDate'.
The unit tests must cover scenarios for all lines of code included in the Apex class, resulting in 100% code coverage.
Run your test class at least once (via the Developer Console) before attempting to verify this challenge.

In the above challege i am not able to write a 'TestVerifyDate' class. I dont unsertstand how to write it for a date.
plz help..