Null troubles with PowerShell AD script for creating new users - powershell

Question

Been smooth sailing with creating users for my domain, now I'm trying to set the uidNumber based on what the last 4 digits of the generated objectSid. Might be a simple solution but hoping for some help.
The rest of the code runs fine until we get to the '$last4' variable so I snipped to make it shorter, but if putting the whole script helps, happy to do so.
Import-Module ActiveDirectory
$firstname = Read-Host -Prompt "Please enter the first name"
$lastname = Read-Host -Prompt "Please enter the last name"
$location = Read-Host -Prompt "Please enter user location (LA/NY)"
$path = "OU=Users,OU=$location,OU=GS,DC=random,DC=com"
New-ADUser `
-snip
Add-ADGroupMember `
-Identity "$snip" -Members $username
$user = Get-ADUser -Identity $username
$objectSid = $user.objectSid
$last4DigitsOfObjectSid = $objectSid.Substring($objectSid.Length - 4)
$newUidNumber = "71$last4DigitsOfObjectSid"
Set-ADUser -Identity $username -Replace #{'uidNumber'=$newUidNumber}
Error
You cannot call a method on a null-valued expression.
At C:\Users\Administrator\Desktop\newtry.ps1:31 char:1
$last4DigitsOfObjectSid = $objectSid.Substring($objectSid.Length - 4)
CategoryInfo : InvalidOperation: (:) [], RuntimeException
FullyQualifiedErrorId : InvokeMethodOnNull

Answer 1

objectSid is not an attribute that Get-ADUser returns by default, the attribute you're looking for is just SID. $objectSid in your snippet is actually null, hence the error you're having.
Also, Substring is a String method and SID and objectSid are instances of SecurityIdentifier. This class does not have a Substring method. You would need to refer to the .Value property:
$sid = $user.SID
$last4DigitsOfObjectSid = $sid.Value.Substring($sid.Value.Length - 4)
A much easier way of getting the last 4 digits would be with -replace which will coerce the SecurityIdentifier to a string before replacing:
$sid = $user.SID
$last4DigitsOfObjectSid = $sid -replace '.+(?=.{4}$)'
Or using -split which would also work for SIDs having less than 4 digits:
$last4DigitsOfObjectSid = ($sid -split '-')[-1]