Fix identification of identifier for error report. Setting it to null if the previous attempt was successfull already obviously was a flaw. Using the wrong key HAUPTPROBEN_NR probably could have been avoided by usage of appropriate constants.

{
    "stammdaten.messprogramm_kategorie": [{
        "id": 1000,
        "netzbetreiber_id": "06",
        "code": "mp2",
        "bezeichnung": "Testbezeichnung",
        "letzte_aenderung": "2015-03-01 12:00:00"
    }]
}