Could you please tell me how to fix an error.
The error is “Cannot validate argument on parameter ‘Identity’.
The argument is null. Provide a valid value for the argument, and then try running the command again.”
Below is overall script.

  $csvpath = "xxxxx.csv"
  $parentgroups =Import-Csv -Encoding Default $csvpath

  $filepath = "xxxxxxx.csv"

  //Get users from nested group
  function ShowUser ($group){

  $dn = Get-ADGroup -Identity $group).DistinguishedName  //Error occurred in this line
  $members = Get-ADObject -LDAPFilter "(memberOf=$dn)"
  $Table = @()

  Foreach($member in $members){

  if($member.ObjectClass -eq "user"}{
  $Record = [PSCustomObject]@{
  SubSecurityGroup = $group
  UserAccount = $member.SamAccountName
  }
  $Table = $Record
  $Table | Export-Csv -Append -Path $filepath -NoTypeInformation
  }
  elseif ($member.objectClass -eq "group") {ShowUser ($member.SamAccountName)}
}
}

//Below is main component
Foreach($parentgroup in $parentgroups){
  ShowUser ($parentgroup.MailingList)
}

I ran each line of the script individually, and it didn’t throw any errors at that time.