FIM PowerShell Management Agent - the object reference not set to an instance of an object

Hi There

I've been running into the following problem when trying to import from my Management Agent, based on Soren Granfeldt's PowerShell MA. In FIM it says "stopped-extensible-extension-error". 

The log contains the following text (and some more which I figured wasn't important for my issue):

07.09.2015 06:11:08: Invoking import script: d:\daten\xml-agent\import.ps1
07.09.2015 06:11:08: Should impersonate: False
07.09.2015 06:11:10: Page token returned: ''
07.09.2015 06:11:10: Custom data returned: ''
07.09.2015 06:11:10: Object(s) in pipeline: 2
07.09.2015 06:11:10: System.NullReferenceException: Der Objektverweis wurde nicht auf eine Objektinstanz festgelegt.
07.09.2015 06:11:10: Leave InvokeImportEntryWorker->GetImportEntries

Der Objektverweis wurde nicht auf eine Objektinstanz festelegt is German for "the object reference not set to an instance of an object"

This is  my import-script:

param
(
	$Username = "",
	$Password = ""
)

[xml]$content = Get-Content -Encoding UTF8 -Path "D:\daten\XML-Agent\Export.xml"
$mitarbeiter = $content.SelectNodes("/Header/MITARBEITER_LISTE/item")


foreach ( $item in $mitarbeiter ){
    $name = $item.NAME
    $vorname = $item.VORNAME
    $persnr = $item.PERSONALNR


    $obj = @{}
    $obj.Add("id", "$persnr")
    $obj.Add("sn", "$name")
    $obj.Add("givenName", "$vorname")
	$obj.Add("objectClass","user")
    $obj
}

This is my schema-script:

$obj = New-Object -Type PSCustomObject
$obj | Add-Member -Type NoteProperty -Name "Anchor-id|String" -Value "123456"
$obj | Add-Member -Type NoteProperty -Name "objectClass|String" -Value "person"
$obj | Add-Member -Type NoteProperty -Name "givenName|String" -Value "Peter"
$obj | Add-Member -Type NoteProperty -Name "sn|String" -Value "Muster"
$obj

I will appreciate any advice to solve my problem.
Thanks