Updating XML file using XELEMENT results in error

Question

I'm updating the XML file with additional data which needs to be stored in the root called "PaketniUvozObrazaca" and it needs to be a new line under </Obrazac1022> child node. Here is my code so far

XDocument doc = XDocument.Load(@"C:\Test\Test.xml");
doc.Element("PaketniUvozObrazaca").Add(new XElement("Obrazac1022",
    new XElement("Dio1PodaciOPoslodavcuIPoreznomObvezniku",
    new XElement("JIBJMBPoslodavca", this.label7.Text),
    new XElement("Naziv", this.label9.Text),
    new XElement("AdresaSjedista", this.label11.Text))));
doc.Save(@"C:\Test\Test.xml");

A snippet of XML looks like

<PaketniUvozObrazaca xmlns="urn:PaketniUvozObrazaca_V1_0.xsd">
  <PodaciOPoslodavcu>
    <JIBPoslodavca>4209862630000</JIBPoslodavca>
    <NazivPoslodavca>PZU APOTEKE LA VITA</NazivPoslodavca>
    <BrojZahtjeva>11</BrojZahtjeva>
    <DatumPodnosenja>2021-12-28</DatumPodnosenja>
  </PodaciOPoslodavcu>
  <Obrazac1022>
some data
</Obrazac1022>
New line goes here
</PaketniUvozObrazaca>

And I get an error message when I execute the code as follows:

Object reference not set to an instance of an object.'

Any help would be highly appreciated.